@karalabe proposed an interesting compromise on Twitter. Read his Tweet storm for the rationale, and other comments on that thread.
Shower thought: ProgPoW is nice, but it runs the risk of a net split. How about a hybrid PoW?
A HF would enable an Ethash/ProgPoW mix (initially 100% Ethash / 0% ProgPoW). Each block could move the ratio by 0.1% in either way. The ratio defines what PoW the next block has.
https://twitter.com/peter_szilagyi/status/1232979860513005569