I know it's a based ebin meme, but enlisting foreigners is literally what led to the fall of the Roman Empire.
It very much was not. The Germans simply took advantage of a long-faltering economy and political instability to end only a portion of the empire (the east ran along quite merrily for nigh another millennium).
Briefly, I'd argue the fall of the west was due to three main causes:
The Antonine plague put a stop to the highly prosperous pax romana, neither demographics nor economy ever recovered. Worse, it halted sorely needed reforms, directly contributing to the next point:
The crisis of the third century, a prolonged period of civil war and political instability. Barbarians play into this, but they're very far from decisive. Diocletian's reforms, while necessary, contributed to the eventual rise of Constantine and his destructive cult, christianity, which is the next point.
Christianity solidified the split of the empire in two and wasted copious sorely needed resources on ill-considered reforms and injustices against the people of the western empire.
This is where the barbarians depose the pathetic excuse for a western government that still remained and set up a successor system. But contrary to popular belief, these barbarians remained ostensibly loyal to the east until finally the plague of Justinian and a prolonged famine crushed any efforts by the east to reconstitute the west. The rot that was christianity had made any such effort futile by this point anyway.
It's a bit more complicated than that. Having non-citizens serve in auxiliaries, with citizenship granted on retirement, was working pretty well with the auxiliaries making up nearly half of the army: What fucked it was the decree that everyone in the empire was now a citizen, so no one wanted to sign up for the auxiliaries.
This led to out-sourcing border security to mercenaries from assorted tribes who didn't feel particularly Roman, which worked out as well as as you'd expect.
Nah. Caracalla's edict wasn't about giving people Roman rights, it was about removing Latin rights. Latin citizens by this time had all the benefits of a Roman citizen, but were free from many tax and military burdens. Forcibly making the Latins Romans dramatically increased how much tax the state could bring in, it was very much a positive act (the state was desperately poor).