Of course, it sucks for several reasons and the fans have every right to be pissed at the players for being cnuts, but it usually doesn't matter for the development of the club, unless it's a choice between staying or going. For Dortmund's development as a club, it doesn't really matter if Götze left for Barca, Real, City, Chelsea or Bayern, because he wanted to leave anyway. Same with Lewandowski now.
Of course it does matter and not only from an emotional standpoint. It does effectively lower the chances for national titles, which are not the primary goals of them but still of course something the players play for if given the chance. Not only lose they quality, but the biggest favourite gains that quality.
In Lewandowski´s case it is even about a lot of money, because according to multiple sources both Chelsea and Real made offers between 30 and 35 Mil €, which Dortmund would have accepted, if Lewandowski would have been willing to transfer. At that point he was already Bayern-bound, though, because they probably promised him enough (salary and cash for signing) to make him unapproachable for any other club.
That left Dortmund with two choices: keep him for another year and let him go for free or sell him last summer for a price Bayern could dictate (certainly way below his actual value), because Lewandowski was already certain to join them anyway and he is by all means a luxury buy and not someone they absolutely need.
Actually, the fact that players of that quality are now interested in staying in Germany really helps Dortmund because they benefit from having a brilliant team within the league. When for example Diego chose Juve over Bayern in 2009, it was a sign for the league not being attractive enough to keep their best players and Bremen had no chance to sign quality replacements like Dortmund are doing now.
You can´t be serious....
First of all the comparision between Bremen and Dortmund holds no ground, because the difference in pulling power is not because of the Super Bayern, but because Dortmund is simply more attractive. They have reached a financial level that Bremen even in their strongest period could even dream of. Bremen simply did not have the same growth potential because their fanbase is way smaller (and always was) and they also don´t have other arguments like a world class coach, a really entertaining play style or a famous stadium.
The Bundesliga did not only gain popularity because of the Bayern (they did, but not the whole league), but because after ages they finally had a serious rival, who could push them. The whole story of the rise from the ashes by Dortmund and them being able to compete with a way bigger opponent despite the financial differences was a breath of fresh air for a lot of neutrals and draw attention towards German football.
Dortmund still has the potential to cause Bayern some problems in their dominance, but they need an extraordinary season and some luck with injuries (because of the differences in squad depth) to do that. Both things did not happen this season and the end result is clear: the title race is over in December. Bayern Munich has won, because they are way too strong to feck up their lead. They can win 2/3 of their games with their B squad because even that team earns more money and did cost more than every Bundesliga team aside a handful of them. Bayern Munich has the financial power to invest more money than the whole rest of the league and did exactly that in recent years to gain their dominance. Their wage roll is higher than the 2nd and 3rd strongest teams put together.
To think that such dominance (both on the pitch and in finances) by a single team is good for the league as a whole is a weird concept. One, if not the biggest question about the outcome of the league is answered before half time. The tension in that aspect is completely gone. It makes the league in that term simply boring.
This is not even that much their fault, but I certainly won´t celebrate them for doing such good work for the popularity of the Bundesliga, because the league does not benefit that much from that. It is actually the opposite.