I can pretty much tell you, at least from my impression, that the core FG developers pretty much only take on areas of FG that directly interest them. So no one will STOP developing some other area of FG just to further work on VATSim integration. That latter work will be done by someone who has that interest at the top of their mind. And it's not like any of the work they might do will in any way hinder the further graphical development, further aircraft development, further sound development, or any other area that's presently underway. Seriously, there's absolutely no harm in it.
To those that are concerned that VATSim capability would replace FG's own MP servers -- I don't think anyone is suggesting this at all.
SP-CEZ -- you're correct that there's much I would have to study up on before considering myself ready to fly on VATSim. But they seem to offer plenty of tutorials on the matter.