I find this point more agreeable than population and time invested. Yes, every rank should be rewarded.
It's a specific reward which OP wants, which is commendation crystals, which are used for glowing weapons and the feast glamour. Right now those are deemed as "prestige" as getting a Diamond/Crystal portrait background, proof of your personal progress.
If I understand you right, you're talking about a generalized, more rewarding system, which doesn't specifically have to be commendation crystals. And when I take a closer look, I suppose the reward from Bronze to Crystal is just a portrait background, basically a "all I got was this stupid tshirt" deal

If there was another reward glamour for reaching at least Bronze to Platinum, would that also be satisfactory? Or does it HAVE to be commendation crystals?
I guess the point I'm trying to make is, if all rewards can be reached through any rank, is it really motivation to climb or continue playing?
Or maybe the "ultimate" reward just be the Diamond/Crystal background, and glamour should stay accessible to everyone?
This is sort of why I brought up Ultimate weapons. Perhaps the weapons can be more easily achieved, but the legend titles go to those that made that end goal.
I think another fair point is that it takes months for a season to finish, which means months until you can receive your reward, and this is really unacceptable for a monthly subscription game. You should be able to receive your reward as soon as you get the rank.