Could the PoS blocks be used to determine the blocksize limit / capacity limit change? For example, if the system sees that there's too little coins being staked or too little amount of PoS blocks being generated, it decreases the limit. In other words, could this scheme be used to determine the safe capacity zone of Bitcoin?
The issue with such a scheme is that you add yet another magic number into the protocol, potentially causing rifts as to what the right amount should be.
Also as the bitcoin economy grows, we should expect wealth concentration to continue to dilute over to a larger number of people. Many small holders would not be willing to spend resources to run a node if the probability of mining a block is miniscule. So i would expect the number of coins being staked to decrease as Bitcoin grows, kind of the opposite effect you are looking for.
Also, number of coins staked != number of different ppl mining