So, yeah, the ending to My Hero Academia is pretty shit, but to put into context how terrible this ending is, we're going to have to explain a lot.
To start with, what was this show about?
A young boy who dreams of becoming a hero lacks powers or, in show terms, "quirks" to qualify for hero work.
It's only until he meets his favorite Hero, All-Might, and through an act of reckless bravery, All-Might trains to be his successor.
The 2nd episode even ends with Deku stating, "This the story of how became the world's greatest hero."
When people first heard this, they said, "Oh wow, he's going to train to become a hero like All-Might. That will be so cool to watch." It created this expectation that he would grow up to be a fantastic hero on par with Superman, and at first, that seems to be the case.
He trained to prepare himself but still struggled to control the powers of One-For-All, but he learned to control its insane strength over time.
He didn't win all his fights but inspired people around him.
It seems to be on the right path, but as the series went on, the story just became a mess.
It got even worse in the final battle with Shigaraki (who, in reality, was nothing more but All-For-One new skin suit, but that's another story for another time). Deku lost All-For-One permanently.
For 8 years, he spent all his life as a school teacher.
All the while, all his classmates were living more successful lives than him and hardly bothered to spend time with him.
He's not even married to any of his previous female classmates.
But don't worry. Through the power of nepotism, he gets a power suit and can join his friends in doing hero work.
This ending is insulting because it wastes the audience's time.
All that hardship and broken bones Deku suffered to control One-For-All was for nothing.
Even if you compare it to when All-Might finally lost his powers during his big fight with All-For-One due to a severe injury, and it only got worse the more he used his quirk when he finally beat All-For-One, he at least did it when putting away dangerous villain and least guaranteed a successor.
Only for said successor to lose all his powers in his final battle with All-For-One.
And you know what? I wouldn't even be bothered with Deku losing his powers if he at least attempted to be a hero without them.
You could even have him go through a whole journey, realizing you don't need a quirk to be a hero.
There are other ways to achieve this.
Hell, the story could given Deku a power suit early on in the story, and while its strength was nowhere near as strong as One-For-All, Deku would make up for it using his smarts.
This could even inspire people without quirks that they can become heroes.
Deku could have basically become Syndrome but good.
He doesn't even need to be one to make the suit. He could meet up with students working hard to find alternatives for ordinary citizens to protect themselves even without quirks.
But nope, he gotta wait 8 years for that suit.
Didn't even bother to ask for one or study to find ways to be a hero without powers.
No, he sits on his ass doing nothing.
It's insulting for the reader to spend so much time reading chapter after chapter to see the protagonist become a wage slave.