It just occurred to me that if the intention is to slowly raise the level cap over time (i.e. through expansions.) then secondary jobs for classes could become relevant.

It no longer becomes an issue of my "Class X" is 50 so my "Job X1" is also fifty and the new "Job X2" will also be 50. But rather (if they do it well.) The level cap is raising to 60. My level 50 "Class X" can continue leveling with "Job X1" to the new cap OR I can unlock "Job X2" and play as that. Or for people with no interest in playing a given class because of the job that it currently turns into, with the advent of new jobs a class may become one that certain people want to play as.

I still think they would need to drastically change their approach to how jobs work, but really there's no evidence that they haven't done that already.

Trying to make definitive statements at this juncture is just a little silly. It's not possible to answer the question with the information provided. Everything else is just assumption and speculation.