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.