Say you took Captain America and said "OK.... No flag-suit, no shield and for a while no big LEADERSHIP! speeches or patriotic words of any sort, we'll change what you look like, but you'll get this cool plasma rifle and psychic blasts."
Would that be Cap?
Or say they took away Wolverine's claws, healing factor, charming personality and body hair but gave him electrocuting knee-pads and a bandana of truth. Would that be Wolverine?
No. It wouldn't.
Everyone's complaining about Doom in the latest FF movie (and the other movies, to be fair). Because he isn't Doom. If you say you're giving me a strawberry milkshake and I'm really looking forward to the strawberry milkshake but instead you give me Diet Sprite, I'm going to be disappointed.
They had a character they called Deadpool. But it wasn't Deadpool. That's a bad thing.