The logic for ash being such a signficant safety issue is that it is one of those common mode failure things. If you are in sufficient ash to fail one engine, the odds are pretty high that the rest of them are going to roll back in a few moments. I think everyone agrees that is a big problem.
However,
engine overhauls are not scheduled 1 hour before expected failure, they have quite a lot of safety margin and typically have some type of condition monitoring to detect accellerated wear. A failure due to Ash wear is likely to be at a random time, and therefore the redundancy logic works.
Just to be clear - I am not saying flying in invisible ash has no effect - just that the risks seem similar or lower to lots of other risk.