This is a consequence of opening up a genealogical website to other purposes, in this case to support law enforcement (LE).
People who are now dead or no longer interested in DNA genealogy didn't gave consent to that usage, the T&C at that time didn't mention the use of their DNA for LE at all.
There is no other obvious choice than to go for opt-out as default. While you could see (in the past, no longer as there user numbers have decreased) up to 200 concurrent users, this is obviously a small percentage of their 1 million plus DNA samples uploaded (there will be duplicates for sure for every DNA service where you tested).
So only a small minority is using it and can make an informed choice of giving consent to LE or not.
Yes, that makes the whole website useless to a certain extend, I do agree with the OP. It all could have been avoided by not allowing LE or even cooperating with them and violating their own T&C (which is a legal binding contract by both the users and the owners of GEDmatch.com) lately.
Everyone who would like to help with his own DNA sample to catch murderers, rapist or any other criminal person should go to his police station and volunteer to give his DNA to them! That should be the way and us genealogist can reunite and work together on our goal of finding long lost ancestors, prove our family trees and discover new close/distant relatives.
Just my two cents worth