If one of the referee's goals is to challenge the PCs in any meaningful way, then yes, making things easier for the PCs (by giving them heaps of powers) makes it harder for the referee (to challenge the PCs). In that sense, it very much is a zero-sum game. If the referee doesn't care about challenge, then giving the PCs more and more power is irrelevant.
I mean... is it though? I still have infinite resources and infinite attacks. As a DM I'm doing fine.
But. let's look at this in a more narrow context. Let's look at traps/challenges/puzzles/ect. Statues that charm the PCs, poisonous clouds, locked doors, ect. Do I need to alter the DCs for any of this? No, I don't think I do. Sure, in a few places saves have gotten "easier", the Fighter's indomitable ability can essentially guarantee they will pass a save. But... I never assumed that whatever PC got hit with these DCs failed. I consider the scenario if they fail, and I consider it if they succeed. And if I was really excited about at least one of the party being hit with a curse from the statue... well, it isn't the fighter's indomitable that caused the low wisdom barbarian to roll a nat 18 and pass anyways. So, nothing at all in regards to obstacles, traps and challenges needs to be changed.
Skill checks? Well... more people being more likely to succeed is a good thing! I want my players to succeed on skill checks. Okay, sure, the barbarian can rage and use primal knowledge to roll stealth with advantage for 10 minutes and sneak around... but that isn't really any different than the rogue/trickery domain cleric with expertise doing the same dang thing. This isn't an instance of "players can succeed where they could never succeed before" but "more players can succeed where only a select few could before" which has me really flippin' excited as a DM. I don't think I can convey to you how excited I would be if a barbarian player looked at a situation and said "I am going to activate rage, and go hunting."
So we get down to... combat power. And is it harder for me as the DM to challenge my players? Eh, depends on what you mean by "harder". Do I need stronger monsters, potentially with some more interesting mobility abilities? Sure, might need some of those depending on the party structure. But having an Oni drink a potion of Haste as a bonus action isn't any "harder" for me to do than it is for me to design The Deep Father as a unique campaign villain. Or the insane mage Godendes. Needing to send more powerful monsters isn't harder for me. Actually, it is much harder to weaken monsters if I overestimated a party's strength. Because I can do anything. I can use the Angry GM's advice and build a monster that is secretly three monsters and have it take three different turns because it is off-kilter from reality and just keeps teleporting around the battlefield. And that isn't "hard" for me. I do that kind of stuff anyways, because "This is a giant" is inherently less interesting as a boss monster for me as the DM, than some giant I homebrewed.
And sure, sure, "well that's good for you but what about DMs who don't homebrew?" Well... if I'm not changing how I'm making encounters in response to the party having a few nifty new abilities, why should I assume you have to? Why should I assume that things are now "harder" for you when they are no harder now than they were before? Even if it is a "Zero-sum game" the DM is sitting with a ten thousand point lead, dropping to an 800 point lead isn't going to suddenly tip the scales in my opinion.