Not only Belllingham but Dortmund has given birth to quite a number of talented players and they always sell it when there are teams who dare to offer high prices. Although able to replace with other players, but I think the concept that De Borrusen built was quite brilliant because in addition to pursuing achievements but they also always won business, they were also always able to be the main competitors in the Bundesliga.
Actually Borussia Dortmund can't find replacing for position leaving by star player after selling with fantastic values, before Jude Bellingam sold to Real Madrid have Jadon Sancho sell to Manchester United. After leaving by Sancho, Dortmund can't find right or the save level with Jadon Sancho and they try for promoting many young player but they need time for adapting before have much expensive values in the future. Last sold is Jude Bellingham with fantastic transfer fees above 100 million euro, Dortmund have face with reality difficult in the winning trend last three matches in Bundesliga, have 2 loss of 3 games its bad performance from Dortmund are teams finished on 2nd standings position last season.
I don't know what happen with Dortmund management with believing fully young player without have experience playing regular in domestic league.