You raise a valid point, but I disagree
Even if only the Lieutenants made noticeable progress, that still doesn't put Ichigo on the Captains' level.
Sure, he has the reiatsu to be labeled as "Captain-class" or whatnot, but that's a far cry from actually being in their league. It's like comparing an
amateur street fighter to a
professional MMA martial artist. Ichigo may be able to win his fights, but he's just not up to their standard.
For example,:
- Ichigo cannot do close-quarter combat. If he were to fight someone like Soi Fong (and obviously Yoruichi) he'd lose.
- Ichigo knows nothing of Kido. Had he gone up against Zommari, or Byakuya again, he would lose.
- Ichigo isn't nearly a genius as Mayuri is. Even if Mayuri isn't a combat-type of Shinigami, he would still find some scary way to walk out of a battle against Ichigo
Aside from those 3, there are the
4 Veterans and the
3 ex-Vizards who are clearly stronger than Ichigo (considering they each have hundreds of years of experience and the latter 3 having inner Hollows just like Ichigo). As well as Kenpachi, Komamura, and Hitsugaya.
Hitsugaya may be the rookie Captain of the Gotei 13, but that was before he even mastered his Bankai which he now has. Not to mention he's proclaimed a genius by the Gotei 13 and who Kyoraku claimed would surpass even him in a mere (relatively speaking) 100 years.
Don't get me wrong, Ichigo has high reiatsu and his inner Hollow which make him powerful, but if you take a step back and ask yourself if Ichigo would make a decent replacement for any of the Captains he just wouldn't fit, and I'm not talking about responsibility issues or that kinda stuff.
You even said it yourself, that Captains have been around for over +100 years. Some for even a couple hundred to a thousand years. Had it not been for Ichigo's FGT he would have died along with everyone else. No matter what kinds of power he gains he can't make up for 100+ (bare minimum) of high-class training and experience. His FGT was an exception, but that's not something he can do again.
It's not just about sheer reiatsu. Ichigo has apparently double the reiatsu of a Captain which was enough to shock even Unohana, but Unohaha could still easily bend Ichigo over and probe him like a stuffed animal