Could they have designed these things while advocating for & considering the GM along the way?... Sure... except
zero of the nine UAs demonstrated such a thing & the last decade hasn't exactly been a glowing endorsement of the idea that "what about the GM" was a significant design concern.
No we know that problematic PC abilities can not be solved with monster design. The last decade has nicely demonstrated that particular fact. The GM has been free to solve problems like AREB yoyo healing & so on with endless force immune helmed horrors & antimagic spewing beholders, but they can't without killing their campaign because adversarial monster design used to shut down players leads to an adversarial game if used for more than the rare noteworthy encounter. Likewise regular waves of quicklings moving 100ft making 3 attacks with +8 to hit plus evasion & more per round on each could solve a lot of other problems instantly... freeing up time spent preparing to gm for the next session while looking for a new group of players being one of the obvious results. That's not even something new with 5e.
The GM is 100% impacted by the rules present in the PHB for players and the PC options available alongside those rules. Pretending they can be severed and handled in different books written with exclusive focus crashes into that impact. There is the very significant problem related to the fact that the DMG/MM should not be used as an errata dump for fixing the PHB via some kind of day zero DLC available months later just because the PHB was
too much "could we" & not enough "should we".
I just went over the expert classes looking to see if the DM was advocated for at all and found the following.
"Making an Ability Check requires you to take an Action unless a rule says otherwise. Several of the named Actions—such as Hide and Influence—include Ability Checks.
The DM may override this requirement and allow a particular Ability Check to be made as part of a Bonus Action or as no Action at all." - Tells the player that the DM can override the rules.
"The DM determines the Difficulty Class of an Ability Check
and can override a DC specified in the rules." - Tells the player that the DM can override the rules.
"
The DM determines whether a d20 Test is warranted in any given circumstance." - Tells the player that only the DM can have a d20 test happen.
"
The DM may determine that other things count as Difficult Terrain based on the examples here." - Tells the player that the listed items are not the only way to achieve difficult terrain and the DM decides.
"To give this assistance, you must be near enough to the ally to assist verbally or physically when the ally makes the check.
The DM has final say on whether your assistance is possible." - Tells the player that the DM has final say.
"The adventurers need to succeed on one or more challenging Charisma Checks to convince a Hostile creature to do anything on the party’s behalf;
however, the DM might determine that the Hostile creature is so ill-disposed toward
the characters that no Charisma Check can sway it, in which case the first check fails automatically and no further Influence attempts can be made on the creature unless its Attitude shifts." - Tells the player that the DM can just say no to the attempt.
"Similarly, if the interaction is particularly irksome to the creature, the DM might impose Disadvantage on your subsequent check
or might temporarily shift a Friendly creature to Indifferent or an Indifferent creature to Hostile." - Tells the player that the DM can just ignore testing for influence and just shift the creature's attitude.
Those are more clearly written examples of DM authority over the game than are in the 2014 PHB or even the DMG. That looks like DM support to me.
Edit: The UA 8 which is the most recent one I have similarly has language indicating absolute DM authority over the rules.
"When choosing a new form, you may look in the Monster Manual or elsewhere for eligible Beasts
if the DM permits you to do so."
"Worn equipment functions as normal, but
the DM decides whether it is practical for the new form to wear a piece of equipment based on the creature’s size and shape.
And so on.