The one thing I will say - and I hate to come off like "that player", so apologies in advance - is that Plenary DOES actually see some use in some of the higher ends of raiding content. In O4S in particular our WHM got a ton of value out of it, and I can think of a few areas right now dealing with Kefka/God Kefka where you might stack heals like that. It's not entirely useless, and hell, in some particularly bad groups in Rabanastre you could also use it to get a clutch heal off (even at just 150 potency, the lowest possible value) on someone who ran in the opposite direction of you when they needed healing.
For example, how often do you predict an Astrologian will get maximum value out of Lightspeed except in fairly niche situations? Here's how I use it currently: I rip it before an Ascend if I don't have Swiftcast up, pretty much just to save mana, or if there's double raises that need to go out (Lightspeed, Swift the first, hardcast the second). No one needs raising? Okay, I rip it on a random Aspected Helios cast then immediately click it off because odds are I'm going back to DPSing and Lightspeed actually lowers my damage output. In a few SUPER rare scenarios, like O4S Almagest (ironically, the same case for WHM and Plenary) I'll rip it to spam-cast some AOE heals to power us through the dot, or during periods of high movement to make my positioning slightly easier but for most general raids I'm using it just to lower the mana cost of a single ability. Its primary boon of use basically increases the worse your party is...does that mean Lightspeed should be deleted/removed/replaced with something completely different? Or do we accept that some abilities will just be relatively niche in their use and that outside of the odd QoL (Confessions definitely need to last longer than 10 seconds on a target, oof) they're fine the way they are?
I mean, I'll be honest, Sleeve Draw as the Astrologian capstone ability doesn't feel that great either, and Minor Arcana is basically a consolation prize for an unavoidable crap RNG gimmick that we have built into our kit. With XIV's current feast-or-famine healing design when it comes to cooldowns and designs there's not really much else they can do or add in terms of abilities, IMO. I feel like we'll really see this crunch when 5.0 comes out - and I wouldn't be surprised to see every class only get 1 new ability altogether just to try to minimize how hard it is to add things to our current healers that actually feels "new".