There is a very uncomfortable reality when it comes to test realms - the realm is not really for the player - it is for the game company. The PTR is not meant to give us a meaningful gaming experience - although some do enjoy it. The intent is to gather data based off filled player testing slots. When those slots are full, the server just queues anyone else wanting in. We are not “entitled” to a slot on the test server.
I know that sucks. But understanding what the server exists for, and how they are primarily gathering data, helps people adjust expectations.
What would drive an expansions in test server capability, is a need for more data that Blizz is collecting. Most of that data is collected by the server, not our feedback on the forums. If they need more slots filled to get more data, they will expand capacity.