It would certainly ruin the difficulty of the FF14 fights, considering how most of them play out with very few random elements.
I think I still take issue with the overall "bigwigs and DBM" play the game for you though. I've been a raid leader for too long to know that things like WA, addons or even a raid leader telling you where to go doesn't mean people are actually going to do it. Tunneling is certainly a thing. I watched friends do some ultimates before and despite having people physically call out abilities that were going to happen 5 seconds before they happened.. they still wiped because somebody didn't listen to their call out lo.
On that topic I'm not sure every ability would be able to be translated into an addon for FF14 anyways, due to how the encounters are. I suppose on a fight like the second one in savage where the boss either envelops the front of him, or everything behind him would have such an addon read the ability and say "front or back". At the same token I don't know how something like intemperance on savage would be able to read in game objects like the floating crystals to dictate whether you stay, move etc. I'm generally not a person in WoW that really used addons anyway, even when doing hard content, except under the rare exceptions where something was so outrageously designed that it was stupid not too (Mythic Azshara, Mythic Archimonde).
Regardless, people will take the path of least resistance and I know a lot of people will have a second monitor up with layouts of positioning (reminders) for certain parts of some savage fights. I've seen a few PF descriptions have links to them, so it's not a stretch that more organized groups with statics would be doing the same thing. Which is kind of like having an addon, even though it's pretty rudimentary lol.
At the end of the day though how this game handles addons is for the best. FF14 actually has a very good raiding game IMO, even if it's handled different than other MMOs. I think the only thing I take issue with on the raiding front in this game (and it's not unique to this game), is that some fights have terrible visual clarity, and some of the markers/colors they use that appear above your head somethings emphasize "style" as opposed to readability.