This is how team's win. They sign guaranteed stars.
Morata may or may not become world class. I'm one of his biggest fans, but he's still very inconsistent.
The lack of resale value is only an argument for selling clubs. Clubs like Bayern, Real Madrid, Barcelona, Chelsea, ManU, PSG, etc don't think about resale value when buying a player in his prime. Especially a top class CF. If resale value is our worry, then we will be forever buying 20-24 year olds with potential, hoping they turn out for the best.
Barca bought 27 year old Suarez for 85 mil. A 27 year old with a crazy history of long-term suspensions. But one of the best players in the world. It's likely, he'll be worth very little 3+ years later in his 30s.
Cavani isn't Suarez level, but he's a top 10 CF in the game, easy, probably top 5 when in form like right now. Paying 40-50 mil for him would be fantastic business, if we get 3 years of him banging in 25-35 goals a season in all comps.
Morata may never get there. Dybala may never get there.
The problem is that we don't have the money to do this, which is why we are currently more of a Dortmund, Atletico, Arsenal style club. Signing young players, hoping they turn out well. So far, we've been very good at identifying talent and also very lucky. The vast majority have turned out well. But if we want to cement our status as a top 5 club in the world long-term, and sustain European success, eventually we have to be hoping we grow in economic power to the point where we can afford 40-60mil attackers, in their prime (25-29 years old) attackers who will have little resale value 4-5 years down the road. Players like Cavani, Sanchez, Aguero, etc. Ideally, you sign in the younger part of that bracket, 25-26... but the younger players that are established already as world class cost far more, as you get more years out of them, or can sell them at 28-29 for a good amount still.
Cavani would be ideal if we had the funds. Ideal.