I'm quoting these out-of-order, apologies in advance.
But in the case of deleting insecure BTC, it seems possible to me, especially if quantum computers and coins being "un-lost" start looking more like a big looming threat and less like a theoretical discussion, that there could be sufficient support for the action that the risk of any substantial hardfork effort splitting Bitcoin would be low.
I thought OP_RETURN was sufficient for creating provably-unspendable coin? If an early adopter wanted to, they could choose that option to nullify their coin, right? Perhaps it hasn't happened because if it did, it would vastly affect the price of coin today. Perhaps Satoshi or other early adopters who are holding their coin without any intention of actually spending it would rather see adoption by those with faith in a more-decentralized asset before the reality sets in and the coins are destroyed outright, thus drastically affecting price? Or perhaps, it's another ironic-and-now-embedded reference to the bailouts of 2008 as the genesis block so alludes, with each passing day a big middle-finger to the existing financial system that just as the large fiat bankers hold the keys to destroying the modern global financial system, so too does one entity now hold the keys to "destroying" the system that will eventually replace it.
I don't think anyone would propose that recovered gold from shipwrecks in the 1800s be destroyed instead of being allowed to re-enter the market. Perhaps with abandoned coins as a prize, significant advances in quantum computing would provide their own just rewards. This could be the intent of NOT making early coin provably-unspendable, though I would echo your concerns about how drastic of an effect on price this could potentially-be and would hope that these owners react accordingly. Worse still, it has always been a sort of unspoken yet lingering concern for all of us that Bitcoin is the work of a nation-state bent on usurping another by devaluing Bitcoin at a predefined time, in which case the owner nation-state would almost certainly not approve of having this early coin programmatically-locked.
We knew these risks and got involved anyway. We've known with a far more accurate estimation since SDL's blog posts in early 2013, and yet the price rose anyway, though it had fallen for a bit after the revelation.
I'm not sure that the existence of *insecure, unmoved coin* is a sufficient precedent for disqualifying coin outright. There is likely some coin that hasn't moved since before the addition of encrypted wallet.dat files that could similarly be argued to be "insecure" at some point in the future (though from the perspective of the network itself it is just as secure). Should those owners from 2009-2011 pre-wallet-encryption also eventually be forced to move their coin or lose it entirely?
I think we should wait and see. Perhaps we will see a solution arise without intervention.
For example, China could right now freeze all the bitcoins of people who haven't been ID-verified by the Chinese authorities.
This is just a nitpick, but I think it is important to distinguish between China -- the country and its people -- and the Chinese government. The pools being focused there can be somewhat distressing, but the people of China may yet embrace the
"spirit of resistance" that Thomas Jefferson once hoped all Americans would have towards their own government if it became tyrannical. So, too, may the Chinese possess a healthy distrust of their own government's adoption of tyranny for the sake of liberty. I do wish they understood that Bitcoin's fungibility is just as important as its role as a medium of exchange.