I think a specific part of PBE has forgotten why they're here. This is specifically targeted toward ranked. You're not here to gain LP. Your goal isn't to hit diamond. Your job is to be playing for the purpose of finding bugs and reporting them to the forums. Now, I understand that some people who play ranked may actually realize this, but play because of the draft mode included. That's understandable, many like the power to ban champions they don't like or have a standard pick order. So, what to do about this?
Currently we have 3 groups. The first group, the testers, actually do their job. They may go in a custom alone with bots to try something out. They are also for the most part active on the forums.
The second group, the casual players. These players mainly play normals. They play to get away from live and chill. Many of them are still active testers and report bugs.
The third and last group, the ranked players. The last thing on their mind is to find bugs. All they want are a shiny border and lp. Ranked players are among the majority who do not report bugs. They want to hit diamond/challenger to somehow prove that they belong there.
Instead of having the PBE population split up into categories, why not combine them? Remove ranked or any other game mode that does not need testing at that time. Leave customs and/or normals open. This will result in faster queue times for everyone and get the point across that you're not here to hit challenger. Maybe even have specific game modes available on certain days. For example, ranked on mondays and normals on tuesdays.
Keep in mind, I categorized these groups of players from personal experience. Do not take this as statistical evidence.