In the context of your original statement: Much less important, but kind of related:" there will only ever be 21 million bitcoin” is a much cleaner economic coordination point than “there will only ever be 19.87 million [or whatever fractional number] bitcoin”

With regards to that ☝️it's the same. We already don't know how much has been lost/burned. It's estimated to be at least a couple of million Bitcoin. There will still be 21m if some are frozen, just like there are 21m even though some are lost/burned now. It effectively doesn't change the total coins. I would even argue that if the owner isn't bothering to migrate them to a segwit address, let alone a future QC resistant address, they're most likely already lost/burned.