I think Barca and Madrid have proven that a two-horse league is the best for Europ performances. You have to perform on a consistent basis because your main competitor will not drop many points but you can rest your players before big matches because no one other than them really challenges you. Bayern reaches the CL final twice when BVB were on their level and have not done as well in the span of time since BVB declined. When the rest of the Prem was weak, the top 4 dominated Europe and since then too many clubs are at the top and the middle is too strong.