That manager, when asked to do it in 90 days, if s/he was competent should have said: I'll do it, but you'll have to accept a downtime risk.

That is the correct take in general, but I've worked for managers a bit like Elon before, and that never would have worked. It would have been the equivalent of tendering that resignation, because he sees any pushback at all as insubordination, and not to be tolerated.

The only way out of this is to suggest an alternative course of action, but make it seem like it was his idea all along. My favorite method was to tie it to some bullshit metric he previously set for no reason. "Yes, sir, we could do it right away, but it may have an impact on our ad throughput, and I know ad revenue is a key prioriry you have personally set for the company. If we take the time to transition our ad platform first we can keep our revenue consistent". Then that time ends up identical to the amount of time needed to move things correctly.

It still might not work, but at least you've stroked the Boss's ego, which can keep you employed for a few more months while trying to find the exit ramp.