I know I have omitted many but the fact is that Dortmund doesn't want to build for long. They are simply doing what they are used to doing and what they think gives them results.
Dortmund are comfortable with their current condition where their profits are only centered on the sale of players they do so I think by looking at the condition of their financial benefits then it has become a comfort zone for Dortmund and they don't want to move from it.
Currently as long as they (the fans and management) are not too demanding with achievements and trophies I think Dortmund will continue like that and there will be no change.
But indeed in this case in terms of business of course they will be more able to expect this as a business field for their financial benefits.
Yes, In general the income they get is none other than the results of selling players in the transfer window by selling star player. This season dortmund will not give up on winning the trophy in the bundesliga even though they need extra energy to compete with bayern munich but I am sure they will be able to put up a lot of resistance even though in the end they will give up again.
Yes, What you say is quite true, Dortmund is always unable to compete with Bayern munich in the bundesliga. But over the past few season, Only Dortmund has been able to provide strong competition for Bayern munich and other teams are actually not that strong when compared to dortmund.