The league isn't in decline, but it's in danger of going on another similar one to what there was between like 2005 and 2010, when Bayern, despite being the best german team, were going into the Europa league because of getting knocked out of the group stage and getting pushed aside easily by the other top teams. If Bayern takes every top player in the Bundesliga, they will walk the league by 30 points every season, and people won't be that interested anymore. Players won't want to go there too much either if it's so one-sided. The league as a whole suffers which would make Bayern suffer in the longer run. Every league needs to be a good competition to survive and stay at the top, and there needs to be big rivalries for titles like Dortmund and Bayern now, and what there is in every other league.
Bayern will be the top team (or up there at the least) in Europe for 2 or 3 years probably but after that there could be a sharp decrease in the quality of the league if they have no competition.