What were the devs thinking?

[Nubrication]

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 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

DRG, DRK, NIN AND SAM can absolutely kill without party members. It's definitely more difficult in Crystal Tier where everyone knows how to guard and purify properly; and when to run from a fight and elixir. In lower ranked CC, casual CC, FL AND RW, they can easily solo kill people (and yes, SAM can do it without LB). SAM's burst window is 40k dmg LOL, not counting their stun (which does 10% of target's current HP) nor any extra dmg from Chiten dmg return/Chiten's 25% dmg bonus). If you can't kill someone with just SAM burst, then it's time to pay more attention to who just used guard and has low mana (in CC).

As for DRG, you're probably not bursting the right people properly if you can't kill anyone. Same with DRK and NIN.

Even in "DUEL" scenarios. Where you both start at full hp and full mana, it's possible. But there are variables that come in to play like environment, HP kit, and elixir. And chances are you'll rarely be in this situation unless you are completely ignoring objective because you have a hard on for someone. This is all of course - in terms of CC. In FL, why would even bother trying to solo anyone. Everybody has innate damage mitigation - and even then it's still possible to solo kill (as those classes) with BH5, etc. (granted it's gonna take you a long time and only if the enemy decides not to run).