Well obviously you'll still sell some players. Barcelona sell players (thanks Barca). Everyone sells to some degree. I'm just talking about selling players you really don't want to sell. I was down with selling Fellaini because the money we got was over his value to our team in RM's system.
There aren't many spots on our first 11 (goalie aside) where you can realistically see a better player wanting to come here. So we buy potential. We have some money from TV to use. If we want more money then we sell. However when you sell a finished article to fund potential you are obviously in the short term making your team worse for potential long term gains that may never be realized. Lukaku was a very different proposition. Thanks to the loan and a great season a player that normally wouldn't consider us now would. So raising money to make that purchase was perfectly logical. In the absence of being able to sign someone like that it becomes an entirely different proposition.
When you have a "well that's __ sorted then" player ... those are the ones I don't want to sell. Stones means one CB slot is sorted. Why sell? McCarthy, Lukaku, Baines, Coleman etc. all represent a spot on the field that's sorted. We likely couldn't get a better player to take that spot if we wanted. Fellaini didn't sort anything out -- he was a bit of a square peg for RM so it was fine to sell. McCarthy was the player he hand-picked to fill one of the (if not the) most important role in his system. Very different situations.
You also have to consider the reality of waiting. Sure it'd be nice to say "let's sell some players to fund young players so in five years when Stones and Lukaku are hitting their peak we have a great team around them." Is that optimal? Sure. Will Stones and Lukaku be here in five years if we don't improve? Of course not. So planning for a future that will never come doesn't seem logical. Sell to buy teams get stuck on this perpetual cycle of rebuilding. You can't rebuild forever. At some point you have to to try to win.
