Agreed, though I'm not sure Bitcoin's users/miners wouldn't "vote" to eventually invalidate unmigrated wallets. There's a tradeoff between accidentally destroying someone's legitimate wealth (e.x. a nLockTime'd transaction left as an inheritance) vs allowing crackers to eventually claim potentially enormous abandoned wallets (worth billions, if not trillions of dollars in the future). But remember those unmigrated wallets would also be equally vulnerable to cracking as abandoned wallets.

Perhaps there could be a solution whereby a nLockTime'd transactions could be presented during the migration period. I haven't thought through the details.

Either way, treasure hunting in the future will almost certainly be of the digital variety.