The Old Republic lasted for 25 millennia, of which -- at most -- 2000 years can be qualified as "a seriously bad time" for the galaxy.
Naturally, anything as big as a galaxy is always going to have some regional issues, but that's basically like saying that if there's a local brawl in some random county of the USA, that "proves" that the USA is a failed state on par with, say, Somalia. Obviously, that's bullshit. Bottom line: for the vast majority of the Old Republic's long, long existence, things were Pretty Damn Great™.
The only real bad times were Pius Dea period (1000 years), the Old Sith Wars (50 years) and the New Sith Wars / Draggulch Period (1000 years). The latter was more of an interspersed period with very dark times, but also some pretty mellow periods. (Or, for another comparison: trying to depict the period as a millennium of darkness is like describing the 20th century as if Hitler had been active for all of it.)
So. 2000 really shity years, tops. Out of 25.000 years. If you can keep existing for 25 millennia, and do a pretty good job of it for over 82% of the time... you're pretty much the best government that's ever existed. Which, in the SW universe, the Old Repulic most assuredly is.
People bitching about the Old Republic completely lack any and all perspective. They paste the most outlying bad times and the exesses of the Republic's literal dying days onto all of its history, and dumbly pretend as if that's fair accounting.
The most retarded take on galactic governance that can possibly be entertained, of course, is the absolute turd-brain notion that the Republic failed in being too much of a loose confederation. The opposite is true: that's why it succeeded. Because every single "bad" period in galactic history was demonstrably caused by attempts at centralising power. Without exception, it was a bunh of centralising tyrants who threw the galaxy into chaos, whereupon peace and prosperty was restored by overthrowing them and restoring decentralised, confederal governance.
As I've argued before: this is because decentralised governance is the only way a galaxy can be ruled in a half-way decent manner. Is there going to be corruption here and there? For sure. Is there going to be bad stuff happening locally, because (essentially) "Planets' Rights"? Yup. Is it better than the alternative? Absolutely.
Because on a galactic scale, with hundreds (if not thousands) of species with wildly different cultures and societies, you can either say "local autonomy!" or you can stomp on them all with a ruthless jackboot, for ever and ever. Those are literally the only options. There are no workable in-betweens, because if you try to limit local autonomy, many will just choose to secede. And then the Republic fails. (So then you watch it fail, or... the jackboots again.)
The only way to keep a galactic government that is not a genocidal tyranny is to make it so decentralised, so localist, so utterly unassuming that nobody really objects to it. That's the Old Republic. That's the best you're going to get, but if you should be that lucky, you can actually have the least bad government there is --- which is the smallest government there is.
Long live the Old Republic, and fuck all centralists. (In the ass, with a hot poker.)