There is no doubt that Dortmund would have been an unstoppable force in European football if they had players like Robert Lewandowski and Erling Haaland on their team. However, I think that is why they are able to generate such a large amount of revenue, because they are able to scout out the finest young players and sell them for a decent price.
For sure there is no doubt that they would be been unstoppable in the European if they keeped there good players.
But if they would have done that i guess also they would be the most bankrupt Club after Barcelona too , its not possible to handle all the salaries without Investors.
And they sadly have not big investors like some english clubs have.