What were the devs thinking?

[Divinemights]

this isnt really the case as some jobs are very independent i.e NIN,DRG,SAM,DRK other jobs like PLD,SCH, and BRD require your team to understand those toolkits to better optimize these jobs utility, so the OP is on point

Frontline wise.
None of the jobs you listed has the ability to obtain a kill without a party, especially DRG, DRK, and NIN.
SAM LB is just a lol and this job has lost its burst ability from 5.1; unless you are counting on inexperience players or tunnel vision players, but fact this job can be just ignored because its damage output can't even suppress Recuperate's CD.
This job is highly rely a group vs group encounter; otherwise I can stand there and let this jobhit me for 15 mins and it barealy can lower my hp by 5%.
DRG needs DRK to setup for them.
Nin need other jobs to lower target's HP below 50% for them.
PLD can be ignored this expansion after they took nerf the cover.
SCH and BRD and supports and not many on Primal even bother to play support role on a support job; unless you are telling me Aether is different
The ecosystem is simply different for 6.1 compares to 5.1