Timo, I think you may have misquoted me.
I hope not... Yes, I left out half the paragraph, but only because I was going to speculate along opposite lines while agreeing with the hard facts in the part that I retained.
Spock would naturally do things properly if he were to actually resign
, but my interpretation was that Kirk jumped to conclusions there, and misread Spock's intentions, and no resigning had actually taken place...
Does that mean despite serving on the enterprise he would always be stuck at a low level position because he skipped ahead?
I doubt it - why should he be stuck at any specific position? But unless he takes the academic studies at some point, he can't get the commission to officer rank. He can only become a higher-ranking enlisted man.
It's relatively rare for anybody who enlists to actually pursue the studies needed for getting a commission. The very point there is that the person didn't want to become an officer, a line of work very different from those available to enlisted personnel! But long-serving enlisted men may be "mustanged" into commissioned rank on occasion; this seems to have happened to Janice Rand, who was enlisted until ST4 at least, and then was seen wearing the insignia of a commissioned junior Lieutenant in ST6. (The actress also played a character with Commander insignia in between, but that probably wasn't Janice Rand, as such a career development would make no sense.)