And if the first should get heavy problems, it becomes not super likely the next wave of people step instant in

The next wave will just be more careful anons. The cost of running the witness server itself is not very high (helped a lot by optimizations since the Hive fork). Even if there isn't an easy way to cash out people will still do it to accumulate stake.

I don't think it needs to be instant. If there is a bit of downtime or slow block production due to missing witness slots, that's okay, the chain will still work/recover.