I mean all of this is not surprising at all. My two cents are as follows:

The devs designed healers focusing on easiness to pick up and accesibility disregarding the long term engagement and what the playerbase that actually played the role on a regular basis wanted, this of course makes that existing playerbase gradually leave the role to be replaced by new healers, but since the role is known for its accesibility and is not fun in the long term (or basically as soon as you become mildly competent) there is not a big number of said new players that play it enough to the point of reaching savage/ex level and out of those there is another group that only does because its a "chill" experience, experience that this tier with its hard hitting content does not give and as such those people do not play the role/play it that often, leaving us with the shrinking playerbase of veteran healers + the small group of new healers that sticks to the role enough to be able and willing to clear.

Add some external factors like how the previous tier was so easy and boring that drove a lot of players out of the role, how Sage is no longer the "new toy" and people are realizing its a Sch clone, how the biggest check for this tier is the mitigation and healers have not that much agency over it yet have to pay the price if its not done properly leading a lot of them to just play with statics or not play at all, how people tend to blame healers for any death despite not being at fault... And you have a really good combination to get a declining playerbase, no one wants to do an underpaid, boring job that gets blamed for things out of its control after all.