Seneca, Lucan, and Petronius, three notable authors of the era, were essentially condemned after the failure. So perhaps some butterflies on literature result due to this. But as for Piso, he didn't seem to be too bad of a guy. If he quickly cements rule over the Empire, then there's no Year of the Four Emperors. His family had numerous relatives like Lucius Calpurnius Piso, Governor of Africa, or the relatives of Crassus. Given people like Lucius Calpurnius Piso Licinianus, there would be a lot of relatives in high places to adopt as heirs if needed.
It doesn’t matter if he was a good or a bad person, which, by the way, we don’t even know in his case, Piso simply lacked the prestige and military backing to ever establish any sort of rule.
Why oppose the Pisonians when they've simply removed a tyrant?
But since Nero is dead, then what? Piso has his initial faction, his relative, the governor of Africa (Rome's breadbasket), other potential rivals of Nero (who OTL created the Year of the Four Emperors), etc. Why oppose the Pisonians when they've simply removed a tyrant? Nero supporters might exist, but through the Senate--Piso's base--Piso has a source of legitimacy to start his rule. It's very possible it might be a bloodless coup leading to the previous dynasty being replaced by the Pisonians (who knows how the adoptions might go, but a link to the Crassii is likely). There's no reason any regional leaders need to revolt like in 69.
Was Marcus Licinius' massive wealth still in Crassii hands at this time? because a smart Pisonian-Crassian emperor holding this amount of wealth could vastly improve certain aspects of roman infrastructure and politics through out-of-pocket donations alone (maybe even passing major reforms with the help of bribes?)who knows how the adoptions might go, but a link to the Crassii is likely
Was Marcus Licinius' massive wealth still in Crassii hands at this time? because a smart Pisonian-Crassian emperor holding this amount of wealth could vastly improve certain aspects of roman infrastructure and politics through out-of-pocket donations alone (maybe even passing major reforms with the help of bribes?)
Then why didn’t Vitellius just let Otho rule? Why didn’t Vespasian just let mVitellius rule? Even if Piso were to have the whole Senate backing him, without the support of an army, which Piso didn’t have, other generals, who had the support of their own soldiers, could attempt to seize the throne. They were used to being loyal to Julio-Claudians, anybody else? They would have seen him as another tyrant for their own propaganda.
The Pisonians were a relic of the republic, with an undistinguished career throughout the empire, thus they could hardly hope to inspire much more loyalty and reverence than Galba did, and Galba at least could claim to have been Livia’s protege, sort of. Claudius became emperor with the sole support of the pretorian guard, Otho became emperor with the sole support of the pretorian guard. Without its support, Piso could hardly hope to get anywhere, and its support was rather fickle.
Also, without the support of the prefect of Egypt, Africa’s support, which couldn’t even be taken for granted (his relative wasn’t even involved in the conspiracy IOTL) would matter little, since Egypt back then was the real breadbasket.
The Pisonians had a broad front of conspirators to push Nero out, including some among the Praetorians. Given the Praetorian role in Year of the Four Emperors, the Praetorians more or less could back a single Pisonian emperor. Nero suddenly dies, Piso rules. Calpurnius Piso Galerianus, his son, is the immediate successor. His brother in law, Lucius Calpurnius Piso, was OTL a supporter of Vitellius as Governor of Africa where he met his end. Now if Nero dies, the Pisonian conspiracy becomes quite eventful.
I will point out that in 66 Vespasian is likely to decide it better to continue the campaign against the rebels in Iudaea than return to Italy. It will not look good for him if he comes back to Rome with an army that was supposed to be avenging the loss of the XII Fulminata's aquilum and is continuing to resist submission to Rome. Removing his troops from the region to march on the city is both a military and political mistake, meaning it likelier for someone--be it Piso, Galba, Vitellius, Otho, or another--to consolidate power. Assuming Vespasian can leave with an army after the Fall of Jerusalem is a certainty, he still is six months to a year too late to have any real hope of taking power.
I will note that if Piso somehow stays in power, the disastrous precedent that the army has the power to make an emperor will not be set. Likely IMO you start to see a process of Senatorial confirmation of the Emperor, often "recommending" that he adopt heirs, making it a more political rather than military situation. Best case scenario for Rome would be Trajan being adopted at some point and the adoptions continuing as or like OTL, but with Marcus Aurelius managing to somehow persuade Pompeius to take the throne on his death, avoiding the debacle that was Commodus and the move towards military rule embodied by the Severans.
If he quickly cements rule over the Empire, then there's no Year of the Four Emperors.
Then why didn’t Vitellius just let Otho rule? Why didn’t Vespasian just let Vitellius rule? Even if Piso were to have the whole Senate backing him, without the support of an army, which Piso didn’t have, other generals, who had the support of their own soldiers, could attempt to seize the throne. They were used to being loyal to Julio-Claudians, anybody else? They would have seen him as another tyrant for their own propaganda.
The Pisonians were a relic of the republic, with an undistinguished career throughout the empire, thus they could hardly hope to inspire much more loyalty and reverence than Galba did, and Galba at least could claim to have been Livia’s protege, sort of. Claudius became emperor with the sole support of the pretorian guard, Otho became emperor with the sole support of the pretorian guard. Without its support, Piso could hardly hope to get anywhere, and its support was rather fickle.
Also, without the support of the prefect of Egypt, Africa’s support, which couldn’t even be taken for granted (his relative wasn’t even involved in the conspiracy IOTL) would matter little, since Egypt back then was the real breadbasket.
The conspiracy happened in 65, if Nero dies, nobody is going to appoint Vespasian for a revolt which occurred in 66. The real man who’d have the most chances of becoming emperor would be Corbulo, he had it all, noble family, a prestigious career, a standing army, and by careful diplomacy he could hope to sway the other generals once the provinces to his cause.
No, the Pisonian Conspiracy was discovered in 65. I am taking it as implicit that, for the conspiracy to be successful, not only in killing the Emperor but more importantly in taking control of the city in the aftermath, the conspirators have to take their time building up their network and buying the right people to their side. I can't find any sources giving the exact timeframes within the years 65-66 the events took place, but depending I don't think it unlikely for the Conspiracy's preparation and the actual coup to take up time into early to mid 66. By the time it is complete and Vespasian has a full enough picture of what's going on in Rome, it seems likelier than not to me that he'll have his hands full dealing with the Revolt. Indeed, one of Nero's last acts ITTL might be ordering the revolt's suppression. Even if the timeline isn't stretched out that far though, it's still unlikely Vespasian becomes Emperor, since to the anti-Neronians he would likely be seen as too pro-Nero whilst nobody in Nero's inner circle thought much of the guy. That, and he didn't have an army until 66.