There's just too many escape valves. If you're a miner, you don't care about what network you're mining, you care about getting the most profit out of your hardware. There are several EVM PoW chains that are established, have been around and are listed and traded somewhere already, and have functioning contracts in them, and beyond that there are other non EVM PoW chains to direct your hardware at if you're running GPUs. It just doesn't make sense for a miner to take the risk of backing a new chain over moving their hash power over to an existing one.

At some point I'm sure they'll remove the legacy mining code, I just don't know whether they've done it yet.