"Long term projections" meaning, projecting a language's popularity in a decade or so?
The SO developer survey and others [0] [1] [2] have been around long enough to notice trends, but projection - whether in economics, sports, or the weather - is of course about much more than trends.
Personally I would seek out the advice of as many trusted greybeards as I had access to; experience counts more than math and statistics. Also, if your research is for the purpose of guiding current decisions (you didn't say this explicitly but I picked up an implicit need) be aware of black swan events [3].