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The JVM can actually perform this optimization optimistically and can undo it if the assumption is violated at runtime. So Java's 'everything is virtual by default' approach doesn't hurt. Of course relying an a sufficiently smart JIT comes with its own trade-offs.

Optimization means "make it faster without changing behaviour in ways I don't like". Clang can't generally default that one to on because it doesn't know whether you're going to splice in more code it can't see at runtime.

Lots of code gets slower if it might need to be called from something not currently in the compiler's scope. That's essentially what ABI overhead is. If there isn't already, there should be a compiler flag that says "this is the whole program, have at it" which implies the vtables option.

I think you have answered your own question: If turning on the setting is scary for you in a very localized project at your company, imagine how scary it would be to turn on by default for everybody :-P