I'd prefer this to be optional to avoid unnecessary sprout (and Eureka/Bozja) death.
If a mounted low-level player/sprout accidentally aggroes a high-level monster (or pack of monsters) and is trying to escape, they might panic and hit the wrong button, then auto-dismount and be eaten alive.
It could also be an issue in Eureka/Bozja where you may have to run past monsters who can aggro if they turn at the wrong time or you get too close, or if you cast (magic aggro) or have low HP (undead/blood aggro.) Casting could simultaneously auto-dismount and cause magic aggro, which could be unpleasant.
I always hate it in Elden Ring when I get knocked off my horse and then have to run away on foot and re-mount, but in FFXIV you can't remount if you have aggro, so it would be worse.