#4 Extra funds for reshaping the squad
United require sizeable investment in all areas except goalkeeping. Therefore, selling him when he would still command a sizeable fee could free up a substantial amount of wages which could be reinvested in other areas of the pitch, most notably in defense.
Real Madrid are said to be interested in de Gea, with Zidane reportedly not convinced by Courtois and PSG are also said to be looking at him to replace Buffon, so the 28-year-old should not be short of suitors.
De Gea will still command a huge fee despite his poor season, given all his records over the last few years for both the club and Spain. United can then use the fee to strengthen their fragile defence and beef up their midfield. Thus, considering the reasons, Solskjaer should sell him this summer.
Conclusion
David de Gea has been a loyal servant to United. The club could not afford to lose the Spaniard in the previous years but following a combination of factors such as his loss in form, his contract situation and the rise of Dean Henderson, it has made the prospect of him leaving seem not so gloomy for the first time.
De Gea could get the chance to move back to his country and compete for top honors which he deserves, but United cannot realistically offer for the foreseeable future.
Selling him would bring United the extra funds necessary to reshape the squad. It has been a long time coming, so this would be the right time for both of the parties to part ways.