PQ signatures are significantly larger than regular ECDSA signatures used today. This proposal builds on BIP-360, which suggests a witness discount increase to mitigate this (not precisely specified, but external discussions by the author have suggested 16-64x). A discount would mean larger blocks, while no discount would mean higher transaction fees and even lower on-chain throughput. Are we entering a new blocksize war? A 64MB block size, in exchange for defense against a vulnerability that at this moment remains hypothetical, seems unlikely to pass without some controversy.