Airlines offer better service if you pay for it. If you choose not to take advantage of that offer, you're telling the airline that you prefer to save money than have better service.
You have every right to complain, but complaining itself doesn't do anything. If you say X when you complain, but say Y when you pull out your wallet, airlines are naturally going to listen to Y much more than X.
I've lost count of how many times I've seen a conversation go like this:
"Airlines suck, I'm a tall guy and my knees are crammed into the seat back for the entire flight."
"Almost every airline these days offers an upgraded economy class with more legroom."
"Yeah, but it's too expensive!"
If you don't want to pay for it and would prefer to suffer (I'm a tall guy, and that's what I do too) then fine, but don't be surprised when the airlines listen to the signals you send with your money.
