Maintaing a list like that would take a considerable amount of time and resources for Let’s Encrypt. Can you propose how to find the money to hire people dedicated to discovering and maintaining such a duplicate resource?
Based on my reading of that PR, the maintainers aren’t being negative. Rather, they are saying that being added on the PSL isn’t just for bypassing Let’s Encrypt rate limits. Being on that list also causes several other things to happen like browsers not allowing cookie sharing and taking other protections. If a regular domain is added, it can cause all kinds of problems for the owner of that domain and it would be impossible to immediately reverse the inclusion as not every user of the list updates on the same schedule. They simply want the operator of the .ru TLD to confirm that org.ru, .net.ru, and pp.ru are public prefixes, much like .com, .net, .org.uk, and com.ar are. Also, it looks like they are asking that the PR be adjusted for the addition to be put in the right spot to keep it clean and sorted. IMO, that’s not much to ask of someone who is responsible for running that TLD.