...free-to-spend coins in the legacy (pre-SegWit) blockchain...
This is pretty old big blocker FUD. If this would be a real issue, why is there a segwit address with 190000 bitcoin? 35hK24tcLEWcgNA4JxpvbkNkoAcDGqQPsP
Are the miners stupid or why don't they grab these AnyOneCanSpend bitcoin?
Because they are
not stupid.
Because it only works once.
Because tomorrow, there will be 19000+x BTC in segwit addresses.