So I guess I'd say my point is that you say "I'll mine, so that won't happen" NOW, which is both fair and rational. But let's say tomorrow we discover a critical vulnerability in the bitcoin source that's been overlooked and causes the whole thing to be destabilized and, effectively worthless. Everyone moves on to BTC 2.0 (or whatever the fork is). Will you still mine original BTC? No. Why would you? It'd be a waste of resources.