Am I missing something or did you not read the original post? That is the key point of the whole proposal. To be precise you can't force someone to migrate per say but by disallowing the spending of previous signature types you are in practice doing just that, forcing them to migrate or lose their coins.
Yes, apologies. What I meant is that some of us are not in favor of a "mandatory migration", but rather an optional one. To not freeze any coin, but leave them on the fate of their security.
FWIW the community can accept addition of new QC-resistant cryptography and address alone, but later reject follow-up change/fork they don't like. In past, SegWit got activated but SegWit2x never got activated[1].
Before anyone start thinking about making any forks, new things should be tested first. Which means for example deploying some kind of "quantum testnet", where people could see quantum-resistant algorithms, try to break them classically, measure how many transactions can be confirmed, and how quickly spammers will post JPEGs by using "OP_CHECKQUANTUMSIG OP_NOT" or similar scripts, and so on.
Looking at past, Bitcoin developer already does such thing. For example, there were test network for SegWit called SegNet[2]. I don't remember whether there were dedicated test network for Taproot or other changes though.
[1] https://bitcoinmagazine.com/technical/now-segwit2x-hard-fork-has-really-failed-activate
[2] https://bitcoinmagazine.com/technical/segregated-witness-deployed-on-new-bitcoin-testnet-segnet-1452277172.