Unless Paradox have radically changed their policy, Closed betas are not paid. So a Closed Beta cannot be compared to a professional testing program. Contrary to popular belief there are not dozens of teenagers prepared to work to a fixed program of work for nothing 16 hours a day.
As I mentioned before - previous experience would suggest you would be lucky to retain 10-20% of the initial Closed beta testers post launch. For the Paradox betas I was involved in you tended to only get between 5-10 active testers operating after the first patch (judging from the bug reports / forum posts).
I'm legitimately surprised to hear this, but I suppose long-term motivation is an issue. And likely it's not very effective to keep recruiting new people, then filter out those unfit for the job, plus the fall-out you mentioned, and then trying to have some form of steady number of testers.
At times I forget that not everyone is obsessed with video games and used to devoting the entirety of his spare time to playing (and discussing) such...
I suppose in that light, I may have more harm with this whole thread, by ending up on a blacklist that will prevent me from becoming a Beta tester for any PDX product ever again ^^'. Then again, I didn't consider the possibility of applying for a Closed Beta I wasn't aware existed in first place (under the assumption the CB for Stellaris is still ongoing).
How much more do you think would be reasonable?
If the answer to that would be simple, IT would have one big issue less.
I.e. in my current project (buisness IT), we have 8 coders and 1.5 testers, and even working overtime, they can NOT keep up. However, despite having to do thorough tests for every feature on multiple platforms, I think 8 coders would be plain overkill.
The core issue here is that, despite being a rather hefty-budgeted buisness thing, I don't think the complexity of this project can be remotely compared to Stellaris, thus my numbers, whilst providing a raw frame of reference, cannot be actually applied to Stellaris QA.