People in a pug M+ group are not necessarily my “peers”. Also, the decision from these “peers” is unlikely to be fair like you think since everyone has their own agenda.
For example, if there was a peer decision system, I could just party up with 2-3 friends, get 1-2 pugs, and those pugs could be held hostage by my friends and I.
Player interactions are complex and there is a myriad of reasons why people leave keys. It is impossible to have a system that will be fair under all circumstances, and having a bad system is worse than having no system.