If you look at the competitor WoW, you may be able to see why changing jobs / classes mid dungeon is restricted. Some of the players over there queue as a healer or tank merely to skip the queue time. They then switch to DPS and refuse to do the role they sign up for. It is a rare occurrence, but still something to note since a bad experience like this really sticks. In order for it to be a thing in dungeons, they may have to add a role check feature that only comes up when someone within a role has to be dismissed. Something like reassign roles after the kick, then search for the last spot. If the UI can get a store of what gear you use with all of your classes or jobs, it may work.
For the reworks, it's hard to say which ones are needed or not. After playing an expansion for 2 - 3 years, players are likely to want something new to do. In the case for healers, the 90 kit additions may feel superficial for normal content and only see real use in Savage or Ultimate. The other classes are usually getting additions to their rotations rather than changes if you compare to their last iteration. There are more major cases of actual change like MNK getting the Blitz system and SMN losing their DoTs in favor of a more button changing experience with the summons. DRK also used to be quite different in Heavensward and was modified to a version that resembles a WAR with more oGCDs.
As for Remolia, congratulations on making it to 80 as only Conjurer? In truth, you are still making it ultimately harder on yourself. White Mage has the lvl 35 quests to get Regen for starters. When used on the tanks, this spell is so handy to free you up for even higher healing with Cure 2 or helping with damage. Divine Benison shields at lvl 66 for White Mage makes it even better. There are other cooldowns of course, but these are some main spells that make general healing much more efficient. Cure 2 and Medica can be replaced by Afflatus Solace (lvl 52) and Afflatus Rapture (lvl 76) in most situations since they heal for the same amount and cost zero mana. Additionally, they also charge the blood lily to unlock Afflatus Misery for a massive AoE damage burst on enemies. All in all, you are really missing out by nixing the White Mage stone.
The Conjurer class does still get used, but for a completely different reason. Macros that copy ability bars to another blank bar. How this works is more applicable to PC from what others tell me.
https://www.youtube.com/watch?v=qe-Gz_4fZcI