I have many random thoughts regarding the subject in mind so my post is not structured or organised around only one point, I don't believe that one league stays always as the strongest, its relative strength is shown in Europe but its cyclical and never remain more than 8 years IMO so I just want to share a few things about the matter:
So as I said above, I would say that the dominations periods by teams from a same league tend to be a cyclical phenomenon with a time frame ranging from 4 to 8 years, every league had it share of dominance for instance I list here a few notable time frames:
Spain: 1955-1960, 2013-2018
Netherlands: 1969-1973
England: 1976-1984,
An even period betwen Spain and England: 1999-2012( we had 3 winners & 5 finalists for the CL alone, Spain had 5 winners & 2 finalists , but we tend to have a small advantage as we had most of the time more teams in the last 8 and semis).
With the ultimate financial power of the PL we are most likely set to witness another period of dominance from 2019 until when? (Probably another cycle of 4 - 8 years unless la Liga make up ground by conquering the Asian & American Market, and promote teams other than Real Madrid & Barcelona or resort to financial doping again).
Now it's all about money and all 6 English teams have a massive advantage now, the best football managers in the world are working in the PL because clubs give them more resources than they could have anywhere else (or at least as much as PSG, Barcelona and Real can give), having the best managers around with the right ressources defenitely helps.