But betraying the krogan means betraying Wrex, Grunt and all of clan Urdnot.

Not a bad thing if wrex didn’t survive and you had to betray wreave. In fact, that’s the only way to get most of the war assets as wreave doesn’t work out that he’s been betrayed. So in the grand scheme of things it’s prob better for the galaxy if wrex died on virmire and mordin survived tuchunka.

But, that sort of proves your point, you can’t save everyone, well… you can (other than the obvious ash/Kaidan), you just won’t be very popular or have Great War assets

People are saved in more ways than just staying alive