craft-loot-purchase, there are multiple ways to obtain gear so having a system in which the "limit" is based per class/job makes no sense.

Takea look at the current content, if you think you should be able to gear all jobs to i90 at the same pace then you are looking at the game wrong.

i70=starter gear, easy to get, to gear all classes with but you don't have to gear all classes with it.

Relic=Main class combo with your i70 gear gets you througha good bunch of content.
Extreme Weapons=Obtain using another job/class relic weapon and bam another i90 weapon for another job and not the one you played as.
Coil tier 5=random weapon that is the strongest, but also random so it's hard to rely on it.

With weapons you can see it like this
Relic=Easy
Extreme=Skill
Coil=Skill/luck

Then with dungeons we have the 4 man i60, Philosophy i70, CT i80 and then coil i90, mytho i90

4 man dungeons i60=can be done multiple times, loot is random
philo=no limit to how much you can obtain and can choose gear
CT=i80 gear but you can only obtain 1 piece a week, but you are guaranteed one piece as long as you continue to do it
Coil=i90 gear but you can only do each area once and if your loot doesn't drop then tough luck
mytho=450 cap, takes time to obtain but you can choose what you want.

To an extent the balance is there, if you are seeing mytho as "have to get this" then you are looking at it wrong.

It's completly possible to gear all classes as it is now but you can't expect everything to be i90 at once.

You want to have WAR and WHM geared then do the following.

Obtain relic for the one you play most, then with said relic +1 do extreme and obtain the WHM i90 weapon.

With the 900 mytho you saved getting the WHM primal weapon use it to invest in gear for your warrior

If your warrior is your most geared then when you do CT focus on getting i80 gear for your WHM

In coil if you are doing it witha static make an agreement on looting, if your war dons a good amount of i90 then try to greed on whm gear

It's all about balance, when you take a step back and see the variety currently offered you'll realize it is possible to gear up multiple jobs at once, it's just that their ilevels won't all be the same.