In a 3 class cabin, if business and first are full. You could pretty much have nobody in economy and the flight will still make a little money.
You could charge 50% on top of the usual economy class seat but if business and first is empty you will be taking off with a big loss. Just putting people on seats is great if you are a low cost single class carrier. For established full service carriers it's not the right strategy.
Even some locos now have a "premium" cabin.