Um im not sure what you are talking about, i have done one complete front mission set and working on the second one, plus once in while doing exploring missions and im lvling like crazy. I have gone from lvl 1 to commander lvl 2 in like 2 weeks, and that's with maybe 2 hours of play a day with maybe 3 days in there of 4 hour late nights with the fleet.
Not sure what you could be doing to not lvl. If you do the highest explore quest that's almost half a lvl each time you do it. Plus you get items to work on your crafting.
No mass leave of the game is going to happen, its super easy to level. I could see someone complaining about the easy factor of lvling but not it being hard lol