PoW without difficulty adjustment doesn't work.

The paper makes a bunch of assumptions that work for email but not Bitcoin mining. They assume that sending computer clients will remain slow while spammers can upgrade to fast computers.

In Bitcoin terms, that would be like saying you have to keep difficulty low because you have a lot of slower miners and that they couldn't participate otherwise. No, Bitcoin doesn't work like that, if a miner is slow and can't participate, tough luck, we don't compromise the Bitcoin network security because of that.