It's not that I wish I was there to prevent past mistakes. I just wish I could be allowed to fix the mistakes I see with the power of hindsight.
But there's worse: most of the time they already know. The various issues I balk at were often bothering them for years, and yet fixing it was never the priority. Week after month, there always was something more pressing, generally about some deadline. Such issues often have gone long enough that the cost of keeping the current cruft have long exceeded the cost of replacing it. But this cost is gradual, so the fix keeps getting pushed further and further into the end of times.
Perhaps my biggest mistake is not seeing that such organisations are beyond helping. I keep getting fooled by promises of doing things right going forward, when I should instead extrapolate the past. Either they're already work in ways that I can approve, or at least live with, or I should seek something else on the spot.
That is to say: You can start now. There’s always plenty of new mistakes to make.