That's the point. Most fighters would have to move up a weight division, so mostly no advantage is gained. We still see the same guys fighting each other except at a higher weight = without half killing themselves cutting weight = better stamina = better fights.
There are not many fighters that would stay in their respective divisions. Edgar and Machida are the only ones that spring to mind.
You have it backwards. We would see less stamina out of almost every fighter because they would still try to cut weight but it would be right before the fight. It would also be way more dangerous for them.
You can't just suddenly expect everyone to fight at a different weight they've been fighting at their entire life anyway. It would ruin people's careers. LHW and HW would also be screwed. Probably FW and BW as well.