tl;DR: great champs and big rankups don't really matter anymore because they're being soft-nerfed and made irrelevant in war on a long-term basis.
before battlegrounds was a thing, AW was the only game mode that let our champs shine. yes, pulling champs like star lord and stark spidey was important for labyrinth, and pre-nerf story mode definitely asked for firepower, but AW was the only repeating content that tested the limits of our rosters and the sole reason why many players chased champs and spent money on rankups (prestige champs don't count since they're just a number).
this takes us to where we are today. story content is easy, EQ and AQ don't matter, and everest content is still very rare, which leaves us with AW as the only mode where champions matter (we can treat BG and AW as the same for now).
niche AW globals and the blacklist system have de facto nerfed our best champions and we have no agency over war anymore.1. we can't use most of our top champs.
2. they tell us who to use and who to rank
3. our rankups get banned
4. then we move on to the next global to repeat the process all over again and in the meantime our best champs sit on the bench

quick tangent. kabam is not shy about wanting the chance to nerf herc, and this thread isn't about him, but for the sake argument let's say they wanted to nerf him. hypothetically why can't they just make a custom tag for him and each season give him 90% reduced damage? well, they can because it's their game, but they won't because it's too obvious and know the community would see through it. they also have better ways of dealing with good champs through indirect nerfs, like what they're doing now, and it doesn't just affect herc
but every other powerful champ in the game.
in comes globals and blacklists....
1. globals
globals are becoming more and more restrictive. it used to be that they gave an advantage, but weren't necessary like they are now. globals are so punishing that if we don't use a niche global attacker we die.
2. blacklists
this makes the niche problem even worse, especially in the second season of a global. the champs that we need and the champs that we just spent resources on are now banned, so we have to dig deeper.
3. whitelists
....also known as a long term rotating
blacklist of the best champs. how can a whitelist be a blacklist? because what good is a whitelist if they're incompatible with the global anyway?
herc was whitelisted during decay where most of the defenders were mystics, placebos fed mystic dispersion, and his kit can't counter unstoppable. unusable. he was also whitelisted in the first season of magic thief. again, unusable.

who's going to be whitelisted in the upcoming second season of magic thief? ghost and scorp.

what good is ghost when
each phase can give defenders 2, 3, 4, or 5 indestructibles?
what good is scorpion against skill defenders who shrug or get powered up by his debuffs? zemo, kingpin, killmonger, jabari, crossbones, agent venom, hitmonkey, and blade. not a coincidence that many of the cosmics have armor breaks that hard counter ghost.

the whitelist is marketed as pro-player but in effect it's a blacklist.
the way war is currently designed nerfs not just the typical top few best champs but most of them, and this gives the feeling that NONE of our great rankups matter anymore because they will always be playing second fiddle to average to slightly above average globals.
i want to use my favorite champs, so did when did mcoc become a contest of mediocrity?
niche globals are not always bad. we get to use champs we've never used before and there's some element of fun there, but our best champs that we work so hard to get (necropolis, offers) and rank ups (content, gifting, spring cleaning, july 4) are no longer legitimately usable and just sit there collecting dust, and it's a crummy feeling.
i have no clue how to balance a game that involves an ever-increasing number of champions and an ever-increasing complexity, but whatever this is, it doesn't feel good and it's obvious what's happening. open up the taps on the globals and let us use our favorite champions again, i can't be the only one that feels this way.