Unfortunately you're being a bit too simplistic with your reasoning for a second combo finisher. Both of the examples you provide when pitted against each other (Souleater vs Delirium/whatever) are instantaneous sources of damage with calculable potency values per GCD; players will use whichever combo mathematically gives them the most value to perform right now, and with both being flat potency per GCD with no other restrictions, one of the two combos will fall to the wayside entirely.
Compare other jobs' branching combos:
WAR's branching combo either gives instant damage and resources, OR extends the duration of a damage buff that facilitates the former.
PLD's branching combo either gives instant damage and resources, OR applies a DoT to the target that's more potent overall than the former.
In both of these cases, there is an element of necessity enforced by time: to use the latter combo more than strictly necessary to maintain their status effects is a damage loss compared to using the other combo finisher, but failure to maintain those status effects in favor of spamming the alternative combo is also a loss of damage.
Now, you don't have to have a status effect involved, but the important thing is the "higher value" finisher has a restriction keeping you from using it all the time.
If you stretch these definitions to their logical ends, you can also see this with GNB: it would prefer to use Double Down or Gnashing Fang over Burst Edge, but can't due to cooldown restrictions.
... None of which apply to what you've suggested, as yet.
My understanding is that it's more of a combination ofI know that Archwizard said that the devs don't like DoTs, but that's essentially what Salted Earth is anyway, as well as Living Shadow. They're just more annoying to use than regular DoTs.
- the devs believe players find upkeep timers tedious or confusing to maintain (especially on enemies, in the event of target swapping)
- bosses can only hold a limited number of status effects (both negative and positive) at a time, which makes DoTs eat resources
So on the one hand, a constant upkeep like Death's Design also falls under both of these despite not strictly being a damage-over-time effect, while Living Shadow and Salted Earth wouldn't since they're external sources of damage that don't have any upkeep (just fire and forget on a cooldown), despite applying periodic damage.
And in the former case, I'm guessing it's no coincidence that NIN and SMN lost their DoTs in the same expansion RPR and SGE were introduced.