tl;dr: RaiBlocks is awesome, but voting is highly centralized (devs control 56%). Proof of Stake would be trivial to implement, and at just 1% inflation per year, support 4k nodes each making $20 worth of XRB per day.
Not entirely sure it needs PoS, it may be another solution but the centralization is a concern. PoS may if badly implement compound that concern, or alleviate it- better to move slowly and correctly than move quickly and in the wrong direction.