Lifelong friends who roomed together at Juilliard, Robin Williams and Christopher Reeve, hanging out in 1970s

Because we can not time travel, I don't think you should spend too much time contemplating this. It is very easy to get caught up in a manic spiral about something that, ultimately, will never happen. No matter how much you wish you could, you will never go back and save Christopher Reeve from his accident.

Also, I believe "Yes," if you know someone is going to be injured (whether they are famous or not), you should do all that you can to prevent it from happening.
There is more than enough tragedy in the world. You cannot prevent all of it, so you should do what you can. = Yes.
There are enough people with spinal cord injuries (long before and after Christopher Reeve) to already justify thousands of doctors and scientists studying the subject even without his famous fall. The reason Christopher Reeve was able to live as long as he did despite his fall is because there were already thousands of people studying the subject and working on solutions even before that day. History is filled with people breaking their necks, and first-responders and doctors becoming better and better at how to deal with those situations over the years.

Reversely, despite his situation, Christopher Reeve used his fame to try to draw more attention to the topic. This is a testament to his strong will, and taking a bad situation and making the best of it. Should he eternally live in a time loop where he always breaks his neck because of the good he tried to do after that (should anyone)?? No, no one should be forced to suffer if it is avoidable. Maybe if he had lived, he might have gone on to champion some other valuable cause that helps humanity (Like Princess Diana against landmines or destigmatizing AIDS patients). Maybe, after Robin Williams passing, Christopher Reeve might have gone on the be a very active champion into more research into Lewy body dementia. We will never know.
We have to take what we do know, learn from it, and do the best we can with that information moving forward.

I hope this reply gives you some sort of peace of mind.
(or at least helps to abate your concerns to some degree)

I miss them both too. Robin Williams was my hero - his death hurt more than any other I have ever experienced. I love him: (from The Richard Pryor Show, Happy Days, Mork & Mindy ~~~~ to ~~~~ Night at the Museum: Secret of the Tomb, and everything in between ー just too many to list them all). They both are missed very much.

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