Proof of Work basically was invented to cope with email spam. That's more or less a stamp.
If you have a KvK in The Netherlands your PII details are public, and you will start receiving physical spam from companies despite such spam not being free to send. I never receive spam from individuals though; only companies.
On top of that, the costs of electricity are not the same everywhere (they are very expensive in The Netherlands, for example, than in the USA, and China is very cheap). Nor is it widely stable available everywhere (examples include India and Africa). Proof of Work is basically a waste of energy as well.
Jk, I just thought it was funny that we pay for services which were always seen as a free alternative to paper mail and stamps were a thing of the past.