Constitution Class ships are in the thick of it.
Either you get some promotions, and a transfer off the ship, or you die.
Logically, you can't get "regular" promotions on the Discovery, until all it's missions stop being highly classified. In two years Tily is going to have have had 4 field promotions from Lorca, return to the regular fleet, and they are going to have no official record of why she is still not a cadet.
Ugh. Field promotions, I get...
if you're already a line officer or NCO
in extremis. But a cadet? And, really, I have yet to figure out why on Earth a cadet is on a supposedly classified, experimental ship. I'm not talking about the real reason why Tilly is there--which, I presume, is to appeal to certain demographics CBS believes, rightly or wrongly, will help keep the show afloat--but the in-universe reason. If she's an
uber-genius, she's certainly not demonstrating it, yet. She's no more or less intelligent than any of the other
uber-geniuses we've seen thus far on the ship, which appears to be the entire compliment (arguably, minus Saru, who, whilst canny, doesn't appear to be written as overly-intelligent). She simply seems to be built from the Wesley archetype and, like the latter, gets the same kind of split-reaction from fandom. Some think she's adorkable, others think she is rather less-than-appealing, personality-wise.
During the run with Burnham, when Tilly took to lecturing her, I actually turned to my wife and said "This is probably the first time I actually felt sympathy for Burnham!" For me, at least, Tilly falls flat and until I find an in-universe reason for her existence on
Discovery, I can only assume it's bad writing to put her there, combined with big data analytics from producers who feel they need to include certain archetypes to round out the cast.
Or, heck, maybe they'll kill her off later on, thus doubling down on the grimdark. Or maybe they just felt that they ran the risk of making
Discovery too dark and they needed comedic relief. Hence...Tilly.