Wikipedia page views for instance show clearly that the numbers are down in absolute numbers (-3%) and the numbers for the English Wikipedia is down even more (-7%).
When you look at the existing research, it is biased towards the biggest Wikipedias and it is biased towards the countries where the most Wikipedia readers are. Arguably this is where Wikipedia has saturated its markets. Improvements made to the Wikipedia experience will help us at best to retain our existing public.
As the "other" Wikipedias have a potential for growth, it is obvious that research can be done to determine what is hampering growth. Growth does not happen because of all kinds of reason; lack of content, not enough quality, no NPOV, it is what people are not looking for, poor network performance, competitors that do a better job. Proper research will find more issues and, research may rank them differently for particular markets.
We do not know what people are looking for in their Wikipedia and consequently we cannot work on providing the missing information. We do not know what prevents people from contributing in many languages so we can not make the right improvements. We do not know if and where network problems make using any of the Wikimedia projects unattractive or even impossible.
This is why we need research. There are universities in every country where students can study these issues for their language(s), their country. There are grants available for such projects. What we need is research that aims to highlight existing issues and help us find ways to address these issues.