They have principles and wanted to make a statement. But like I said in a different reply - my default interpretation of this given the circumstances would be not considering this as more significant than someone earlier in their career simply speaking out. Would they have resigned if they were only 35? maybe - and that would (to me) be stronger evidence of how bad the things were that they were protesting. And maybe not (this would be my guess, but I could certainly be wrong).

We get it. You're saying the same thing over and over. You do presumably realize that, being 70, they have no way of pushing a button and suddenly being 35 to pass your apparently magical test of significance to their actions. Given that as smart as they are they can't invent an age reducing time machine but they did encounter what they consider to be serious issues at the institution where they worked in their 70's and did not encounter similar serious issues where they worked in their 30's, what exactly would you have them do that passes your personal threshold of significance test for their actions?

I would also consider that they want to cancel out any sense of their late-career presence being an endorsement of a place or practices they find unacceptable.

One might almost hear a tinge of regret that they aren't retiring now with a nice party at UC Berkeley....