they needed more heroes from the beginning

Imho, the problem is the reverse. The game had enough heroes, it's just that too many existing heroes were boring, underpowered, or (if they got sufficient power) game-breaking, and so they were too hesitent to say "this power doesn't work with our format, we should get rid of it". Instead it was tweaking numbers. There's a place for tweaking numbers, but when stuff fundamentally breaks your chosen format (eg. ana antiheal) you have to take a firmer hand. But they let game-breaking powers like Mercy rez ult or just too damned many barriers sit instead of saying "okay, this isn't working, let's throw this bad idea out completely". But of course, gamers scream if their best girl gets touched, so they're really hesitent to make big moves.

Plus, too many of those heroes were damage heroes considering the team can only have 2 of those.

Overwatch was such a good game to watch. I watched some matches with my girlfriend back in season 3 or so. She had never seen overwatch and had no idea what it really was. But she was super into it. I really started to dislike when Ana was released, she was the "you need that hero to win" pick. She hardly ever suffered from any nerfs, and people lose their shit when you even suggest it. When heros like roadhog get too powerful and gets any pickrate at all, he got berfed to the ground. But fine, go on. GOATS killed it for me, it's like you said, 25 hero's, but you would only ever see 6 or 7. And it was just so boring.