I didn't look at the total coin supply yet.
(...) are we to expect 15 minute PoW blocks soon ?
I might be wrong (seriously, I'm not an altcoin expert), but I think so.
When PoS blocks come in every 90 seconds, the POW target spacing should increase until it hits the 15min max limit.
Example calculation without PoB blocks, assuming that PoS "hashrate" (more accurately: the total eligible coin age in all PoS-enableed wallets) is fairly uniform:
| PoW spacing_____ | Expected non-PoW blocks since last PoW block_____ | New PoW spacing |
| 2*90 | 2 | 4*90 |
| 4*90 | 4 | 6*90 |
| 6*90 | 6 | 8*90 |
| 8*90 | 8 | 10*90 |
| 10*90 | 10 | 10*90 (max) |
When there is a PoB block, the "non-PoW blocks since last PoW block" number gets increased by one, accelerating this process.
Thus, the PoW target spacing should indeed tend towards 900 seconds (15 minutes).
However, since sometimes (or maybe even quite often?) there will be less than 8 non-POW blocks during a 15-minute interval - causing the next PoW spacing to go down slightly.
Due to those temporary deviations, I expect that the average PoW target spacing will be slightly below 15 minutes.
If we drop the assumption that the PoS "hashrate" is fairly uniform (for example because some people don't stake 24/7 but just during a couple hours when their PC is powered on), the average PoW target spacing should drop another bit lower because the actual PoS block spacing will temporarily rise above 90s, increasing the probability of getting less than 8 non-PoW blocks during a 15min interval.
