Just some of my own opinions, not justifications for Disney, just trying to think about it from a corporate perspective, and like it or not Disney is a business...
I didn't think OSHA shut these rides down. Yes, OSHA issued citations and fines, but Disney closed the rides down to deal with these issues.
Why didn't they close the rides down before? An incident happened in November, but then there was an outside investigation, probably an internal one too. Disney probably knew something would need doing, but when did they get the investigation's findings? Government doesn't exactly work quickly. From Disney's perspective, there's no point in taking a blind stab at what precisely is going to be required of them to correct any safety violation they have made, so they wait for the report and then take corrective action.
Disney just had a very busy spring break period. Unless they were forced to shut down rides due to immediate safety concerns or mechanical failure, they probably weren't going to do so. If they're not being forced to do so, why did they voluntarily shut rides down now? Perhaps they only now received the final report. Perhaps they received the final report a few weeks ago. In either case, from their perspective as a business, now is the right time to take care of these issues since they're probably hoping to allow themselves enough time to get everything done before the crowds ramp up again for summer, and they didn't want to have too many major headliners down when they were slammed by spring break crowds. My guess is that they didn't have the results of this investigation before spring break because they probably would have used the time between early January and early March to get this done if they had known the citations then.
Another thing about spring break, everything was up and running A LOT. Disneyland was open 8 AM - midnight for two weeks running with an additional hour each of the three magic morning days per week. DCA was open 8 AM - 10 PM for two straight weeks with additional early entry time four days a week. The parks were open not quite as long but for extended hours before and after those busy two weeks. There have been a lot of ride cycles. Sometimes during and after these extended use periods, the rides go down more often for short or even longer periods. Honestly I'm not sure I've ever been to Disneyland and not noticed at least one ride that was closed unexpectedly for some time during the day, whether it was a few minutes or a few hours. So just because a ride is closed doesn't mean it has anything to do with any of these worker safety concerns. Doesn't mean it doesn't either. Maybe they are being extra cautious and covering themselves by double-checking everything. Frankly, I think we're never going to know which instances are which.
FWIW, we went to Disneyland for the first time during the off-season just before the 50th. Space Mountain was on its extended refurb. Jungle Cruise was closed. My kids were terrified of most things and too short for others, so I didn't ride Haunted Mansion, Big Thunder, Matterhorn, Splash Mountain, Star Tours, or very much in DCA. Despite that, over three days, we still didn't have time to finish all the other things we did want to do. We didn't have much money then, so I didn't think we would be getting back to Disneyland a lot, but I still looked at it as a fun family vacation and was glad we had gone. We're now fortunate enough to go a few times a year, so I can't compare my experience to someone who's going on a once-in-a-lifetime or a once-every-few-years trip. We were there earlier this month though and I didn't ride Space, Soarin', Thunder, Splash, or Grizzly and again still didn't get to all the things on my mental list but still had a fantastic time. For those going anyway at times when big rides are closed for whatever reason, planned or unexpected, sure, be disappointed that your favorites aren't available but don't forget about all the other great things that you can do when you're at the parks. For those for whom these types of situations are dealbreakers, vote with your wallet; do something else with your time and money until you feel Disney is doing right by you.