Hard to estimate. But I think failing to consistently reward miners somehow - be it with tail emission, demurrage, or something else entirely - probably would increase the overall risk of Bitcoin failing by something like 25%. Moving to tx fees dominating is a huge state change that no other PoW currency has undergone.

ETH is actually kinda close with how MEV games are dominating block creation... and that's one of the reasons why the supermajority of ETH blocks are now OFAC compliant. It's too hard to profitably get involved in ETH block generation without a lot of highly specialized expertise, controlled by relatively few companies, mostly with US ties. Not good.