I wanted to flesh out the anti-pro Severus arguments a bit more. I think it will help us better answer the op.
The anti Severus narrative largely stems from Gibbon and Alfred von Domaszewski. Gibbon called him "principal author of the decline of the empire" and Domaszewski said he "planted the despotism of the east in the soil of the west". They effectviely viewed Severus as a Punic barbarian, in love with Hannibal, who wanted to destroy Greco-Roman world. They hold him responsible for the decline of the empire, move towards a more Diocletian style rule and the "barbarization" the army.
These views have been countered by Maurice Platnauer and Anthony Birley in their works on Severus. To summarize Severus did not hate Rome and in fact fought numerous successful campaigns in defense of the empire. His reign was progressive in nature and sponsored the legal work of Papinian among others. Severus was also not responsible for the quote "barbarization" of the army as the policy of recruiting from the borderlands began under Hadrian. Severus wasn't of the Italian elite and thus overly criticized as un-Roman . Similarly later Emperor Galerius has been accused of trying to reestablish a Dacian empire and destroy Rome....
I would like to note one thing Severus did that was out of the ordinary was stationing Legio II Parthica a few miles from Rome. The anti Severus group would take this as a sign of his more authoritarian nature, when compared to previous previous emperors, and that he viewed Italy as any old province. Platnauer dismisses this and takes the view that Severus stationed the legion there for strategic reasons. That Severus was against having legions only guarding the frontiers and favored a defense in depth, a precursor to Diocletian's later deployment of the legions.
Thoughts?
I'd say the most uniquely revolutionary factor in Severus' reign was the idea that he was the first emperor to codify into law the idea that the emperor was above the law, which he himself did not abide by as he wished for the public to view him as a law-abiding leader, but this laid some of the first important groundwork for the transfer of legitimacy that would mark the reign of Diocletian and the rise of the dominate. Severus was a competent administrator, a skilled general, and a savvy politician, he was simply despised by the senate for his failure to "show them the proper amount of respect and deference" which is the same criticism they levied against Domitian, Hadrian, and so many other emperors. Domitian in particular is an example of the senate's fickleness with respect to the emperors. During his reign, Domitian fortified the Limes Germanicus and the Danube border, paid for a number of infrastructure projects out of the imperial treasury, and even sat as a judge in a number of civil court cases during his reign (and by all accounts delivered just and fair verdicts where most were concerned). But in the end, the senate assassinated him, damned his memory, and had all statues of him demolished because he openly despised them, which broke with the precedent set by Augustus and later by his father Vespasian.
In my view, the crucial institutional change that came with the transition from republic to principate to dominate was the gradual erosion in the power of the senate. They height of the senate's power came during the middle Republic, at the time of the Punic Wars. They had absolute control of the army and the government as a whole, they had broad public support, yet they deferred to the needs of the state as a whole and would not produce the sort of corruption that would be seen in the 1st century BCE. By the late republic, any charismatic general could strong-arm the chamber into backing whatever agenda they wanted (see: Marius, Sulla, Pompey, Caesar, Mark Antony, et al.). By the time of the principate, emperors like Claudius, Domitian, and Hadrian demonstrated that they could spend years on end administering the provinces without even returning to Rome or considering the senate, and by the time of the dominate, the capital had been moved away from Rome, so the senate could not even raise a finger to interfere with the emperor's administration.
I believe that this institutional evolution reflects the changing needs of the empire as a whole. The republic, as it was originally conceived, was designed to govern *one city*, and they stuck to that governing framework (with only small changes) for 500 years before it ultimately imploded in the fall of the republic. The transfer of power to the executive was a necessary transfer because the threats the empire was facing changed fundamentally after the end of the Punic and Macedonian Wars. Existential crises gave way to frontier defense, and the unwieldy and unstable senate gave way to the more stabilizing guiding hand of the emperors. When you're governing a small republic with a citizen-soldier army, you can afford to keep the kinds of inefficiencies that democracy necessarily creates, but when you're governing a massive empire with a huge tax base and a large, professionalized army guarding the frontiers, you need to have the sort of strategic long-term vision that the fleeting, one-year-terms of the consuls and shifting loyalties of the senate can't provide.
Got a little off the rails there so, in summary, I agree, Severus was one of the more competent emperors of the pre-3rd century lot, and I agree that it's unfair he takes so much of the blame for institutional problems that he didn't create. The fall of the principate was inevitable because Augustus didn't adequately address all the governing needs of the empire when he created the principate. That said, Augustus' constitutional framework did last an impressive 250-300 years (depending on where you mark the line dividing the principate from the dominate), which is still longer than all of US history so far, so we shouldn't be too hard on him.