They are indeed "the" unit at the beginning of the game but people who know how to play the game don't get overrun by them.
I think Mecha-Lenin will beg to differ...some might say he "knows how to play the game"....it is not a question of skill or simply "knowing" how to counter them...they are the early game and if you are even slightly outnumbered there is virtually no hope...
You have options: build your own corvette fleet, start a flak defense, go for a starbase, or [imagine this] scout your opponent to see what he's up to you and prepare your defense.
SB defences will not do you any good against corvette raiding...even if the corvettes can't take on your SB, they'll simply move on to another gravity well and raid there...the resources you plop down to maybe protect one planet will cripple your offensive ability and allow you to get completely walked over by more conventional forces later...
Scouting your opponent does little in regards to corvettes...even if you know where and when your enemy is hitting you with these things, you still aren't going to be able to effectively counter them...flak can't catch them, and can't even kill them fast enough unless you have a ton of flak...
That corvettes is marginalized late game is a moot issue...early game they are the dominant unit and they make 2v1s impossible situations for the suicide player....