Dortmund sells their best players year after year after year and remains a top team.
Admittedly that model is a lot easier to sustain when you're in the Champions League, but the point is that if you're not at the top of the food chain then you have to know when to cash in, and how to reload.
I understand the Dortmund model of selling for a profit and re-investing , it's not new and been around for ages.If we build from what we have and sign 3 quality players a season then we'll need 2 or 3 seasons to get up the table to where these players want to be.
By that time Digne would be 29/30 and Richarlison 25/26 and both will have handed in transfer requests. Dignes value will have dropped also.
If were going to sell Digne then the summer is now with 3 years on his deal.
Hopefully we keep him and find a way around FFP but if the money is good and we replace with 3 new signings its worth a gamble.
In principle , if we have limited funds , it is a decent way to go , provided you don't waste your profit on crap.
However if you follow this philosophy and you get a bigger profit by selling say Holgate or Richarlison , why wouldn't you them instead?