Great discussion @chbartist.
In my opinion, as you alluded to, proof-of-work will always be central to the Bitcoin network as it is the only way to maintain true censorship resistance.
Now that the SEC has unofficially declared Ethereum “not a security”, even though it was initially sold as a security, I would be surprised if it moved to proof-of-stake because this will likely reduce it's decentralisation and put it back in the regulators' firing line.
That being said, the planet can probably only support one proof-of-work cryptocurrency so we'll have to see what happens. On a side note, Bitcoin mining will likely result in increased use of renewable energy and efficiency (to increase profitability) so this may change things.
Like you say, though, proof-of-stake works well for networks like the Steem network and those that don't face as much pressure from governments and regulators.