I think the best way to think about it is to think of older games. Lets take Magic: the Gathering, because there is a surprising number of similarities with this situation.
Magic was released as a free game, and the cards were given away. It was designed well, but a few cards, specifically the cards in a specific cycle, were problematic.
See, the designer had designed a cycle of cards giving each color (there are 5 plus artifacts in MTG) a card that did “something” times 3 for one mana. White gave you 3 life for one white, red dealt 3 damage for a red, blue gave you 3 cards for a blue, black gave you 3 black mana for one black, green added 3 toughness and power to a creature for one mana, and artifacts got the Black Lotus, a card for zero mana that gave you 3 mana of any color. Seemed fair and fun.
Except when Magic started growing, and it grew fast, people found out that it wasn’t balanced at all. 3 life was basically useless in exchange for a card, 3 cards for one blue was ridiculously good, the others were “ok” and Black Lotus was a card that would basically win you the game if you had it in your opening hand. Even now, after around 14000 cards have been released, Black Lotus and Ancestral Recall are the two best magic cards ever printed. So much so that at a point EVERY SINGLE player had to play with them in his deck if he wanted to stand ANY chance at winning a big competition.
So for a while, even though there were five colors of magic cards, you only saw decks with a blue color base and artifacts. Everyone was playing variations on the same exact deck, and it was boring and the game became about who won the coin flip at the start of the match, since he could play his too-good cards before his opponent.
So they made it so you could only play with one of each card in your deck (down form 4 of each).
Immediately new decks started popping up. You suddenly had dozens of viable decks, some that even profited from not winning that initial coin flip (you get an extra card), some that were based on red and very quick, some that were based on green and used green creatures, and the game became way more complex (you had to know how to play against a ton of decks) but WAY WAY more fun.
So why am I telling you this?
Basically, the current Sentry+Hood strategy is the old Black Lotus+Ancestral Recall of Magic. This single strategy is just so superior to anything anyone can do with other characters that the entire game becomes about this. Developers have to dance around the combo - you can’t have a pve node that gives out incredible rewards in exchange of beating it, since people will just Sentry bomb it to pieces and collect loot. You can’t let people use their full rosters most of the time because the combo is too good - or if they can, you have to make the nodes need to be done over and over because that’s the combo’s only limitation - it hurts it’s own guys. In pvp we all know what happened of course - if you want a top10 spot, you need these two specific guys in your composition. You don’t check to see if you’re bringing in Spider-Man because the opponent will do a lot of small damage which you can counter with protect tiles. You don’t slot in falcon for a Dino so you can slowly ramp up damage. You don’t plan for a draining strategy via blade or mystique. You Sentry bomb like everyone else.
TL;DR:
We need Sentry nerfed because he puts constraints in both what the developers can throw at us as challenges and our choice of viable pvp teams, creating a situation where the too-good character needs to be constantly accounted for.
Whenever a way-too-good strategy comes up, it needs to be tweaked for the good of the entire game. Does that mean some day changing x-force’s black? I don’t know, probably. Will people that bought his covers rage? Probably. But if a game is to survive in the long term (Magic has been around for 16 years), it needs to be done.
Edit - that was way more long winded than I intended it to be, sorry
