I have the pleasure to work with quite a few "old" guys that initially enlisted in the US Army just prior to or during the Vietnam War, when the post-Korea Specialist system was in effect. In talking with them I've come to think the Specialist system has some merit. Generally speaking, the idea was that you could have some very senior guys in combat support and combat service support who didn't need to be "officers" of any stripe or bar. They could be experts in their field, and continue to advance in grade even if promotions to sergeant rank in their field were limited.
For instance, one of these old-timers worked in a brigade interrogations section where the senior interrogator was a Sergeant First Class, but all the other guys were SP6 or SP5 based on their level of experience. My grandfather came in the Army as a cook in 1950, and he was promoted up to SP5. It wasn't until he became the Messhall Sergeant that he was actually promoted to sergeant rank, as a Staff Sergeant.
If I were Chief of Staff of the Army for a day, I'd bring back a middling specialist system, much like we had up to the '80s, with SP4 through SP6. I would allow SP6s to serve to 20 years and retire, but I would peg the retention control point at 20 years so they would have to take promotion to Sergeant First Class or retire. Of course, there are those who understandably say the US Army's rank system is too muddled as it is, and to them I say maybe so, but at least it's better than it was in WWI when each corps had different insignia and different titles, not to mention pay scales.
Of course, I'd also not mind the all or nothing approach and go back to the Continental Army's enlisted ranks of private, corporal, and sergeant, with all senior sergeant ranks being appointments much like the Commonwealth nations. But that's probably a different topic.