Okay, here's my thoughts on this:

As the current system works now, I would not agree with you.

Why? Because I actually dislike how people can jump in to duties without knowing anything about a job just because they leveled the other.
Example: SMN player joining an extreme trial as SCH even though they have never played it at all and just bought gear.

If you'd have said that you level together only until you become a job I'd have been on board, but unless I misunderstand you actually want the leveling to continue past the class and into the job, and that I disagree with as much as I disagree with the current lvl smn and get a free sch model. Especially with how different the jobs become as levels increase.

But I see a good potential for capping all classes at lvl x and letting jobs level individually which would increase both proficiency and streamlining of classes/job.

I don't think bringing up the job skip counter-argument makes sense because you only level to 10 levels below cap so you still have to get some experience with the job for 10 levels before you're allowed to jump in to end game content, which is good. Some people learn fast and don't want the full tutorial and that's fine.

But I would prefer linking gladiator to samurai/paladin/dark knight/gunbreaker because samurais are actually in the paladin job story, which I won't spoil here. Gunbreaker being a blade that's enhanced by magic bullets also makes sense to branch it off gladiator. Dark knight is literally the anti-paladin who doesn't follow orders blindly any longer, again won't spoil the story because you just have to experience it.

(continued, post is too long)