We have struggled to shift players of any age for years, or not accepted offers when they maybe should have done. its just the older ones don't have value to other teams so are less likely to be bought than Maddison for example.
The 15 or so first team players that have left for free are not just the over 30s, its the model of signings/wages the club has taken on since the prem win.
I would like to hope the youth are going to become the significant resale value as they progress with the club, as we have a core of a team that looks to be coming through. Sunderland re built from a similar position bringing through a lot of youth, so that would be a good place to start.
I do agree we need a couple more players in attacking positions, which I would like to be around mid twenties who are proven (big ask!)