Hey Rumi, great questions. here are my thoughts:
Q1 On Blur/Tensor: The main reason is that they've been able to ship a great product, and attract the deepest liquidity using their incentive programs. Many people continue to use Blur/Tensor even if there are no rewards, myself included. If OS or ME drops a token, there will be fierce competition for sure, since most traders will flock to where they can potentially earn the most.
Q2: Blur is probably a reasonable proxy for the NFT market (and TNSR when it launches for Solana NFT market). I wrote about this in an Alpha Feed post last week.
Q3: Most interesting to me are consumer use cases. I personally find Blackbird (restaurant loyalty program) very cool, and thinking about how that could scale up.
Q4: IMO Soulbound NFTs' main use case will be in a representation of user's identity and reputation.