>nothing wrong with having rushes and being able to defend against them
Yes there is. Rushes are not played on the same field as the rest of the game, they are a feature you can use that completely change what the match plays like
for the sake of an early victory by the side that suceeds.

The only thing rushes do is place burden of knowledge on top of new players that gotta prepare against weird tactics they don't expect because they are not that intuitive.
Why would you hamper your economy by buildings Barracks right away and start producing troops in the beginning with barely an economy at all?
Surely that will harm your late game?
Oh wait, there's not gonna be a late game, alright...

Either they suceed and the match ends right there with a victory for the rusher or they don't and the massive waste of resources will cost him a victory later on
in most cases as he will be competing against a player that is in a better position to control the map, reducing the match from 20-30 minute into a 5 min one.
And if they are viable at all, they invalidate most content from the game.
Why waste your time making anything other than Zerglings or Zealots and rushing your oponent if that can win you the game?
Why try any of the advanced units when mobbing the basic troops gets you a victory much faster?

Rushes are a subgame that veterans force the new players to play because they don't know the rules (notice how veterans don't try it that often against other veterans)
A subgame that invalidates 80% of the content in the game and turns a fun match with battles all over the map with different units
into a single battle with the most basic and boring troops right at the start.