I would think it will scale just fine in a technical sense up until there are so many posts and things being rewarded that everyone gets payed 0.001hp or something, at which point it kinda breaks.
I would say that when SMTs are released hive will basically turn into etherium.
Hive will end up as the base core currency required to stake in order to run the SMTs and place like hive.blog, peakd and leo finance will make their own tokens and use those to rewards content in their own ways. At this point the hive reward pool could be retired in favour of a basic inflationary stake mechanism or something and basically be used as fuel to do transactions on the blockchain.