Yes, this is a reasonable reason. Even for now, their current transfer record is still in a loss position, with a not insignificant amount, €-22.90m.
Source: Borussia Dortmund if they didn’t sell their stars
Here is their squad list if they do not sell any of their top players so far. However, does not selling them mean Dortmund can be much more advanced? Not necessarily, because once again, Dortmund's finances are not that good to keep those top players, and that is the reason why they often sell their top players, for the sake of their financial balance.
Life is not reverting back and use many assumptions like this. Dortmund succeed with their strategy to build up their team with young players around the world, then sell them with high transfer prices and repeat the cycle. They did not win many titles recent years but maintained their position good enough in Bundesliga and Champions League.Source: Borussia Dortmund if they didn’t sell their stars
Here is their squad list if they do not sell any of their top players so far. However, does not selling them mean Dortmund can be much more advanced? Not necessarily, because once again, Dortmund's finances are not that good to keep those top players, and that is the reason why they often sell their top players, for the sake of their financial balance.
It means Dortmund are growing up sustainably and I believe they will continue with this development strategy than taking more risk to buy star players and pay them higher salary. A healthy finance from sustainable strategy is key of a club future and Dortmund are doing it well.
It's clearly regretful to revert time and look at star players they sold recent years but you can not achieve all things in life, only one or another.