Caveat: I was in the Army Reserves, not the National Guard, which uses a different promotion system. Also, I got out about a decade ago, so it may well have changed since then. That being said…
Yes, I think that’s plausible. The “up-or-out” policies discussed upthread are for active duty military personnel. Guard and Reserves are different. I personally knew a Reservist who retired from my unit as a Specialist (E-4), after twenty years of Reserve service.
In the Reserves (and I think this is true of Guard as well), you serve in a specific slot in a particular unit. The slot has a specific pay-grade. You can be up to one pay-grade above what you are slotted for, but beyond that you have to find a new slot, which often means finding a new unit, in order to get promoted. My memory is fuzzy on this, but I think you also only have a limited amount of time you can serve in a slot if you are above its assigned pay grade, after which you have to either find a new slot appropriate to your grade, or accept a demotion back to the slot’s pay grade.
The guy who retired as an E-4 was comfortable in his role and his slot in the unit, and did not want the extra responsibilities of being a non-commissioned officer. He also didn’t want the extra hassle and time on duty.
You may have seen the old recruitment commercials for the Guard and Reserve with the tag line, “One weekend a month, two weeks a year”. Even in peacetime, that’s only sort of true. If you are a junior enlisted (E-4 or below), in a position without any special responsibilities, you may actually get by with just the one weekend a month and two weeks a year. Beyond that, there are all sorts of schools and extra training, and extra duties at the unit.
As a Reservist, in addition to that minimal requirement, I went to foreign language school, foreign language refresher courses, Primary Leadership Development Course, security workshops, and frequently got called into the unit a day early for the one-weekend-a-month “Battle Assemblies”, and got called in during the month between Battle Assemblies on a number of occasions, was scheduled for Airborne School when our unit was reclassified and we lost our Airborne slots, and was scheduled for the Basic NCO course when my enlistment was up. And I was still a relatively junior Soldier, only an E-6 (Staff Sergeant).