If you take a look at our status in the area51, all criteria seem to be satisfied (soon) but not the number of questions asked (which seems to be decreasing, actually). Do you think this is a problem for us? Is it something we should / can take care of?
This pattern is not unusual at all. Most sites go through a honeymoon period where users pile in from the excitement of a new launch with their questions at the ready. Then you experience a sharp drop in activity… followed by a pattern of slow, steady growth as you continue to compile content and the Google and network effects start to kick in.
This is all completely normal and expected.
I think the fundamental piece is to accumulate users who ask high quality questions and/or provide high quality answers. The "we"/"us" you refer to is not static, of course, and the hope must be that the user-base on MESE continues to increase. How this accretion will manifest is still unclear (to me).
I don't think the concern you are putting forth is that there is a danger of running out of meaningful and tractable questions in Mathematics Education (i.e., as an area of study); but I remark, nevertheless, that such questions are certain not to run out in our lifetimes.