What factors affecting aircraft, missiles sustained turn rate?

I have an argument with someone on another forum, he claims that most fighters cruise at altitude of 30-40K feet because at higher altitude the air is too thin so aircraft wont have enough lift , i disagree, in my opinion that only true if you fly a subsonic aircraft, supercruise aircraft can fly much higher because higher speed will give them more lift , in case of very high performance aircraft likeF-22, PAK-FA, Mig-25 they can easily cruise at 65-70K feet. He continued and said that even though higher speed will give aircraft more lift, their sustained turn ability will reduce too much at altitude higher than 30-40K, making them more vulnerable to missiles. ( he say it because of something called excess power).

so is he right? or iam right? what is excess power?, is it true that missiles can still maneuver at high altitude despite much higher wing loading because they have high thrust/weight ratio?