It is a bold thing to say that Dortmund won't be winning the Bundesliga title for sure in the coming a few years. But I can't deny the fact that Bayern Munich have been dominating this league for 11 seasons straight. Last season was the biggest opportunity for Dortmund to finish their series but they messed everything up in the final moment.
Now it is again up to Bayern Munich's performance level. As long as Tuchel has a good system working for them I will be expecting the continuation of this dominance from where it left. Dortmund can't deal with them while they are showing a high consistency. Dortmund can only benefit from a possible chance in case of Bayern Munich's being out-of-form.
I am not saying that Bayern will definitely be the Bundesliga Champion in the next 5 years too. I just think that Borussia Dortmudn will probably not get a chance as big as they had the last season to become Champions themself. I think that in the last season it was not only Bayern Munich that showed very poor performances for their standards but RB Leipzig also. They had a very bad start into the season and even had to change the coach back in september.
So i think that it is very much possible that Dortmund will not become Bundesliga Champion even if Bayern Munich is not winning it, because other clubs like Leizpig and Leverkusen have a squad just as strong as the one of Dortmund in my opinion.