If that did happen, miners would likely stop relaying transactions from the old address type before it became economical to harvest them en masse.
Why? Also 100.000000000000000000000000000000000000000000% of miners? Even if the tx includes a giant tx fee? I doubt it
Of course not voluntarily. In the scenario where ECDSA has been broken and its use has long since been phased out, there would doubtless eventually be a discussion among the community about whether to make a hard-forking change that after block X such accounts cannot be spent from. This would of course be a massive heated debate that we don't need to have here and now, but it's my opinion that imposing a rule preventing such spends would be for the good of the community, and I'd go out on a limb and speculate that that would end up being the consensus - although if mining hardware at such a time were capable of the cracking operation, miners' personal interests may prevail and a lot of hashing power would end up moving from safeguarding the network to cracking keys.
