Since I appear to be in the minority, I guess it's time for a long post.
I wasn't there when the decision was made, but I get the feeling this was not thought through as deeply as it deserves. If it was, a post that goes into at least as much detail as this is warranted to explain the controversial decision. That is to say, I agree with the decision to automate this behaviour, but not how it was presented, nor what appears to be the logic behind it.
Let's look first at the role of area control plasma artillery. Area control means enemy units will not be able to operate freely in the area controlled by artillery. Anything that steps in will take chip damage and be strongly incentivised to walk out.
A common use, and that arguably fits into the the role of area control, is sieging an enemy position. The previous iteration has provided a lot of control for precision strikes. This is undeniably a form of skill expression that got taken away, and we're not able to replicate a skilled player's targeting choices automatically, nor should that level of smart automation be implemented if we could.
An artillery emplacement is not a precision instrument. Bringing back a toggle without decoupling the stats of the firing modes again will not make it as much of a precision instrument as it was before (which never made sense intuitively anyway, it's a hidden mechanic of sorts). Area control can be executed by a dumb artillery piece.
There's one more use that's been floated around and is not directly related to trajectory, but keeps being brought up in the context of units that use it. Area control artillery are not defensive emplacements. They do not perform well as defensive emplacements, and any artillery utilised as such should be expected to die without accomplishing much. 4 pitbulls will hold a push much better than a rattle.