My only point was that usurping generals used to be a much less problem in the early Principate, particularly with the Julio Claudian dynasty when Italy was still considered the master nation. Once this was lost, regionalism became a big problem. Generals were able to find support of troops recruited from particular regions, rather than from Italy, and these troops held no loyalty to Italy, as opposed to the whole cosmopolitan empire which they were a part of. This would be unthinkable in the early Principate, or even in the 2nd century when the idea of provincials fighting for the mantle of empire was done only in emergencies, not as a regular matter of imperial power plays.
As for provincial emperors, this was the first step towards the idea that anyone from the provinces could thus become emperor, making regional usurpations more likely in the future, even if the early provincial emperors had stable successions. It was a gradual fragmenting process of legitimacy.
One question, was post-Severus more political stable or pre-Severus? Pre-Severus, senators held most of the power other than the emperor. Post-Severus the power balance shifted to the equestrians. Soon you had equestrians trying to become emperor directly. Clearly giving equestrians military/provincial power did not bode well for the empire in the long run, even if it did help the emperor in the short run by marginalizing his enemies in the Senate and giving power to equestrians he could trust.
Politically, the empire was as unstable in the first century as it was in the third, whenever under a weak ruler. If the emperor proved incompetent, or unpopular, the Senate would simply kill him, potentially causing unrest in the meanwhile. If that’s stability to you, to each his own.
Replacing senators with equestrians also allowed the empire to avoid massive disasters like the battle of Abritus, fought by Decius, a former usurper of senatorial rank. Equestrians were generally the better generals, and all the emperors that allowed the empire to recover, from Claudius II to Diocletian, were all equestrians, or soldiers who scaled the ranks.