This shows why Dortmund could not beat Bayern Munich in the last 11 years. They sold most of their best players to Bayern Munich and we see the reasons. Selling best players already reduce their squad power but selling those players directly to their direct competitor is worse. They became weaker and their competitor became stronger, no chance to win Bundesliga.
Well, this is a good observation but it doesn't seem to me as it looks and these transfers are to and from of the two clubs, so it's normal with football. Aside from the fact that clubs' targets are not only about winning, money making is also a good consideration, clubs' speculation could either work or not. And those players that were much younger and not performing well or those that were not performing to expectations might suddenly become a golden booth elsewhere after their transfers. So this is not new and can't be the sole reason Bayern wins Dortmund often even as both are keenly contesting each season. Transfer regrets often happen in clubs.
