I'm not sure Rome fell in 88. I'm not sure it's fair to say even that a hostile army entered the city.

Sulpicius Rufus, as tribune of the plebs, was running the assembly with outright violence, had threatened the life of an elected consul and had driven the consul out of the city. One of Sulpicius' laws was a law distributing Italians across the voting tribes, fundamentally changing the nature of the Roman state. I may be wrong, but I think using the plebeian assembly to transfer a command was unprecedented. Whether or not it rises to the level of a constitutional crisis I suppose depends a lot on your own politics, but the matter was certainly one of how Romans govern themselves, not an imposition by a foreign power.

I think it's worth noting that - as far as I'm aware - no one wrote about Sulla taking Rome with the sort of view that Jerome or Augustine wrote about the sack of Rome in 410 CE. Granted, all the surviving sources post-date Sulla's dictatorship and Sulla's first march would have been, well, if not normalized, contextualized into a post-Sullan view of history, but I don't see any evidence that people in 88 thought it was the end of the world.

If we do think Rome fell in 88, then must we also think it fell to Marius? To Sulla a second time in 82? To Caesar in 49? Several times in 69 CE? What about the ascension of Claudius, secured by the Praetorian guard? Rome starts falling an awful lot.