Jon leaving is the biggest and central GG mystery ever. But I think we might find out in the future, what I feel is stopping us from knowing is the fact that Jon and GG are probably contractually obliged to not speak negatively to each other given they're both from Polaris. But with more recent mentions of Jon from the Grumps, and Ross/Markiplier(two people who were on GG) being on Starcade, it seems the tension is loosening up.
I pretty much like GG mainly for the Jon era. Though I don't have anything against Danny, and again did watch and enjoy some of the newer GGs, just sucks that those newer ones are never finished ;_; then again it's not like it was much better in that regard in the Jon era(Banjo Kazooie and Sonic 06 come to mind..).