I understand that from the perspective of bitcoin maximalists, PoS blockchains don't have enough trust with their censorship and anonymity issues.
Other than that, I think there are other issues. For example, compared to existing alternatives such as using a multi-sig to secure the loan (if it is a collateralized loan), your solution would add another insecure layer. If it is an uncollateralized loan, then the lender can easily solve this by limiting the criteria for borrowers. While your solution can help them reach other lenders, there is not enough incentive to take that risk compared to what we have right now. At least that's how I see it when I lend to somebody here.
The second question, the difference between other lending platforms such as MakerDAO or Aave is that they have strict policies on what collateral on what interest rate you can lend and borrow (pretty much like banks) and usually everything is contained in 1 smart contract, but in my mind, I have an idea where 2 people can create and deploy a lending smart contract with the terms that only both of them agree. I mean, without big web3 companies, they can deploy the separate contract only with the terms they mutually benefit from. For example, the minimum collateral in Oasis App is 175%, but I have seen in the bitcointalk some people are willing to accept collateral as low as 130%.
Your idea is similar to Kanpeki then. Last time I checked, not that many users are using the to borrow/lend, probably because most of the users are just looking to earn more profit by depositing funds to some lending pool instead of manually finding contracts and funding them. Try to check them out and see if you can improve on their model, who knows maybe you can find customers for alts lending.