The logical conclusion of the OP's argument is that, since all value is subjective, any asset that accrues more value over time is a "greater fool" scam doomed

The logical conclusion of the OP's argument is that, since all value is subjective, any asset that accrues more value over time is a "greater fool" scam doomed to collapse. Not a very sound argument. Eventually the Bitcoin network will accrue so much value that merchants will starting demanding payment in Bitcoin, and people will use whatever payment rails are available to make it work. This is how it becomes a medium of exchange. It's really that simple. Most or all of the rest of the cryptos will collapse into Bitcoin.