Yes Dortmund is indeed used to it and have been going through this a dozen of times by now I guess, but they have been doing an impressive job at finding replacements for these super talents. Financially this has been paying off for them for a very long time now, but I understand when they think that they are still facing a loss in terms of the quality of their team and the upcoming season. Once you see that someone has integreated perfectly into the existing team, it would be preferable to keep him if possible. But that is how the business goes. Bellingham won't be the last young super talent that they have to let go.
This trend has been something they have done from previous seasons where Dortmund always make big profits in the transfer market because they sell their star players to other teams. Even for bellingham he is a player who is quite influential for dortmund this season but Management still sells him in order to get income.
From the other side it is true that Dortmund can find the right replacement figure if one of their players goes to another team in the transfer market. For that I don't think they will be strong enough to win the Bundesliga trophy if they do the option to sell star players but that is their tradition.
The sad truth is that Dortmund has no choice but to sell those players. If they decided to not sell and instead keep the player until the contract expires, it would not be sustainable for Dortmund at all in the long run. It is almost like consolation money when they have to let their best players go but at the same time earn record breaking transfer fees in the Bundesliga. I was just going through a thought experiment and tried to remember which players they had to let go in the past that then became world class in other teams. There are quite a lot of those players and nobody can tell where Dortmund would be today if they were able to keep those players and convince them of a future with Dortmund. But the offers from abroad are often too tempting.