I hope for their sake that they never do it
Turning them off unilaterally sounds like a very bad idea, no matter how much advance warning is given. It would be a blow to AWS's credibility. Like any platform, customers depend on it to not break their stuff.
"Don't break your customer's stuff" is just about the first commandment for running a successful platform. (If not, it's definitely top 3.) Every time you break their stuff, customers will do a cost benefit analysis on moving onto one of your competitors instead of moving onto version n+1 of your platform. If you get a rep for doing it often, moving to your platform becomes unattractive.
IMO the obviously correct thing to do is just suck it up and deal with the technical challenges, indefinitely. Charge more per request to the path style endpoints. Those customers will have a financial incentive and you can aggressively market to them "heyyyy you can reduce your AWS bill right now simply by changing how you use URLs." They'll get to it when the savings outweigh the risk & expense.