The thing about Synastry isn't that it's a bad tool, but that it's been severely power crept. The thing about healing is, we keep getting new healing cooldowns every expansion, but the amount of healing required isn't really changing. What this means is that even though we already have enough healing tools to get through all the content in the game, the new tools being some combination of stronger, more frequently available, easier to use, and DPS neutral has gradually devalued tools that used to be very important resources. And it's not because they no longer are important, but because there aren't enough reasons to use them.
Great examples of this would be spells like Succor and Regen. Both were very important to Scholar and White Mage in the past, but have become largely irrelevant because there are just better resources to heal with, and any decent healer is very infrequently pushed to use them anymore.
We cannot continue to just add more and more healing resources unless you increase the demand for healing to match, or we're just going to balloon this power creep more and more. Look at tanks. The demand for personal mitigation has not changed very much across the lifespan of the game. Tanks acquire about 3 extra mitigation cooldowns after level 50. Healers acquire about 10 healing cooldowns after level 50. And I didn't include actions that received upgrades, just new actions altogether in that.
Not every new healing action (or tank mitigation action for that matter) is a bad thing. There is something that can be said about the QoL that certain tools provide, but to the degree that healers get new heals without upping the demand for heals just creates a lot of dead weight on the healer's hotbar. And that's not to say that those dead weight actions are badly designed, or that you don't ever use that at all, but that what they accomplish is now done with something new instead. Why Succor when you can Sacred Soil? Why Regen when you can Afflatus Solace? You could merge some actions, or cut certain unnecessary ones and do a better job of ensuring every action has a place on each healer's hotbar.