Well I told ya I could write paragraphs on this topic, so it’s on you for asking 
. Tl;dr at the bottom though.
I’ve talked about the deficiencies of Valkyrie in this topic. In hindsight I find it incomplete, but the gist of it remains true. I did expand on the subject in this post, in which I also talk about Res and its problems.
About the damage boost: It solidified Mercy’s spot as secondary healer even further, for it makes reacting to boost someone pretty much impossible, you have to boost preemptively, you cannot adjust on the fly, thus a hard pocket is required. Juggling playstyle grew increasingly clunkier and clunkier and is pretty much worthless now.
Healing overall also grew more consistent, even the supposedly inconsistent healers really aren’t anymore. Ana got more ammo, auto heals on ult, Brig has 3 packs instead of 1, Moira has more juice regen and longer duration of the HP/s effect, Lucio’s aura is larger, so Mercy’s consistency lessened in value over the past two year or so.
Add that to role lock, which pretty much deleted an entire niche of comps that supposedly fit Mercy (but even then was played with Ana), and you have a main healer target juggler, turned secondary healer second banana to Pharah, because the first playstyle was pretty beaten into a pulp and no longer fits the game.
The main healer juggler playstyle has been made so clunky and so much worse by all of this, combined with the design problems of Res and Valk, it’s pretty much gone from the game. And it shows, apart from quad DPS memeing comps, Mercy hasn’t been played without a main healer in the pro scene ever since patch 1.27.
tl;dr: Valk covers for far too much skill, clashes with Mercy’s kit. Damage boost change made juggling playstyle significantly clunkier. Changes in healing overall made consistency less valuable.