There are a few problems with Belisarius accepting the Gothic offer working out well:
1. Not all of the Goths were on board with it. Ticinum, where Uraias was based, and Verona, held by Ildibad, both refused to surrender at a time when everyone thought Belisarius accepted. These two were no minor nobles, but men with huge power and influence among the Goths: Uraias managed to persuade the assembly to grant the crown to Ildibad , whilst he became his right hand man. If Belisarius chooses to stay and become emperor, he will have to defeat these two men and the coalition they will surely gather around them.
2. Belisarius' army. Whilst he had shortly been appointed supreme commander for Italy, previous events showed that parts of the army could easily side with one commander against the other, as was the case with Narses. The army itself was also very diverse: you had
i) a freshly arrived force from Dalmatia - never saw Belisarius before in their lives
ii) the heavy cavalry - these men were comparatively well off, and had all their property and families back east (and would not be very willing to give that up)
iii) the Hunnic mercenaries - these had absolutely no problem with B. becoming emperor AS LONG AS HE CONTINUED TO PAY THEM. B himself had no estates in Italy, all his money coming from Justinian's state coffers, and the Huns knew this. Moreover, even if they stuck around initially, they are likely to side with the Gothic nobles (the ones who had the money) once B has difficulties taxing his new subjects in order to pay them
iv) the fleet - I have serious doubts B could force them into joining him. These men probably had some property and families back home in the Aegean sea, and were also aware that siding with B would mean having to face the next fleet Justinian would send after them, whilst remaining loyal carried little drawbacks. If B has no fleet, he will have trouble taking and holding coastal settlements, as happened during the Lombard invasion, when the fleet was instrumental in allowing the Empire to retain control over half the peninsula
v) the local Romans - these men were fighting for their homes, and almost always sided, if given the chance, with the army they thought was least likely to burn down their cities. If B lacks a strong enough presence in the cities, these will switch over to the Empire in a heart beat
3. given points 1. and 2., B will have to fight to secure his rule, and this against both his former country and against the people he was trying to conquer. This will not be cheap, and will force him to levy very harsh taxes. This alienates both Goths and local Romans (making them liable to switch sides), and also leads to 4
4. Belisarius killed many a Goth at this point, and there were probably quite a decent number of Gothic nobles willing to assassinate him at the drop of a hat (assassinations were quite a thing - both Ildibad and Uraias succumbed to that fate)
This is why Belisarius accepting the Gothic offer of emperorship doesn't lead to peace, but to more death and suffering, which WILL weaken Italy. This will most likely entice the Franks to invade afterwards as OTL, further weakening it. At that point, the Lombards will look across the Alps and see easy pickings - the only chance the local elite will have to keep their possessions would be to either side with the massive Lombard host or seek the Empire's protection. If Belisarius is miraculously still around at that point (he would be approaching his 70's), it would be game over for him