I looked at the question you linked as an example; and to be honest, I don't see a need to remove that. In fact, I'd argue it's better than the average PokeBase thread, because it has good, well-reasoned responses, and an interesting premise. These are the same qualities I want to promote in this community overall.
I can see how that whole post is a bit pointless, and how it's difficult to answer 100% conclusively. But I don't have an issue with either of those things in principle, so long as there are actual arguments you can present to reach a conclusion for the question. I actually think that characterises the answer you wrote.
However, this is only one example of a 'theorymon' question, and there are certainly others I don't think we should allow. For example, if someone designed a completely new Pokemon and wanted to know if it would be viable, I wouldn't allow a question about it. That's where the whole discussion becomes entirely speculative, and the most correct answer turns from yes or no, to a tentative 'maybe?'
I can't make a hard-and-fast rule for where I draw the line with this -- it's just another one of these times where things are left to mod discretion. However, I am currently drafting a couple new threads which should give some structure to rules like this, so stay tuned for that.
But for now, in short: if the question sounds very speculative, then it isn't allowed. However, if good and decisive arguments can be made about the question, then it's fine.