Agree with that sentiment. Of course you sell a player or an asset when they/it is at its maximum value (including potential value). But iro Arblaster, he is only just starting out on his career, is tied to a long contract, and is currently happy to be a Blades player. He has vast amounts of potential and the value in the market (I.e. the most ge is ever going to be worth) is in all probability, only going to go up from here with the trajectory of his career.
Unless there is an absolute necessity to be selling the lad from a financial perspective, then we would be absolutely mad to contemplate selling him unless a totally ridiculous offer comes in and blows us out of the water. There is said to be 3 x PL teams interested in the lad at around the £20m figure. There is a reason for that as they can see his potential and know they could potentially buy an asset who is a young technically gifted English lad with vast potential, who they can use and then flogged for a real profit in thr future if he goes on to do what they think he will do. If Archie Gray (who I was never overly impressed with personally) goes for £40m then we really need to stop thinking that £20m for our own young star who is captain material in the future, is only worth £20m. He's worth a lot more and the 3 clubs interested know it.
We should be telling them it starts at £40m or do one. If we are still in the Championship when he gets down to 2 years on his contract then maybe at that time we seriously start thinking about selling him for an optimum price. But right now you keep your main assets if you want to get back to the PL.
It amazes me just how many see a few £ signs and want to flog our crown jewels. I'm fine with player 'churn' but only when we have had proper use of that player and we have enjoyed watching them play and they are then sold at a premium. Barring a serious imjury, Arblasters value in the market is only going one way. Keep him.