I was thinking about class/job also...and it's not really terrible.
Class is basically your skill with using a particular wep, like Lance(er) As a Dragoon, you use spears too, and any other job they may add, who use spears.
If they removed class system, then you'd have just DRG, and other spear Job...but it wouldn't make sense if you suddenly didn't know how to use a spear, just because you're another job.
You go to LNC guild, and become a trainee, and learn how to use a spear; and regardless of what spear-job-stone you equip, they both are based on your skill as a lancer.
It's not the jobs that lv up, its your class that lv up. The better lancer you are, the better dragoon you are. You use your lancer skills to overcome challenges, and unlock the power of the dragoon stone. If you didn't train as LNC, then all you'd have was jump...that wouldn't be effective...
If they add another spear job, you don't lv up that JOB, you're already an accomplished lancer (if you capped) so the new job use those skills too.
They may increase lv cap, and you might get lots of good job skills, and end up not using LNC skills, anymore; but they are still the basic skills, you learned as a lancer trainee.

There are problems though ofc.
Such as with GLD...it's really not gonna be possible to branch off a DD from GLD, because it's so heavily focused on tanking. If they branched off into a DD, then the DD would have access to:
Rampart, 20% reduced damage taken
Convalescence: increase healing received by 30%
Awareness: cant get critted
Sentinel: reduce damage taken by 40%
Bulwark: Increase block rate by 60%
Fight or Flight: increase damage by 30% (stack that with B4B and/or Raging Strike)
You'd basically be both a tank and DD...and that's quite OP.
The only way to fix it, is to rework GLD from scratch, with multiple jobs in mind (which they didn't, when they first designed the classes) They did with ACN though, which is why it wasn't to difficult to branch into a healer and a DD.
They could ofc scrap classes, but then we'd have the issue i mentioned earlier, and also: If they had made SMN and SCH, without class, then they would probably have designed both SMN and SCH differently. SMN maybe wouldn't have been a dot based job, and who knows what attack spells SCH would have had. Point is, they would have had to make 17 unique skills, and 8 traits, for each job. Which would be 2x more than what we have. And why do that, when (in this game) both SMN and SCH, have the same basic training? The only thing that sets them apart, is what they learned from their respective Job Stones; and that's just a bonus repertoire to ACN skills.

Also, they will increase lv cap, so you'll need to lv again.
Also, they will add Rogue, which means you'll need to lv that from 1-50 (or soon to be new lv cap, whatever it might be) And they'll add more classes (they will have to, cause most of the current classes, are poorly designed, for multiple jobs system)

Those dead content you mentioned, are easily beaten nowdays. Find some FC, and it won't be difficult to get a bunch of ppl, to take you trough. We took a new person through primals, and T1-4, in one evening.
Even DF/roulette, can fairly easily get through primals/T1-4. And there's PF too.