For the last 6 years or so, American women have been competitive in the 1500 and getting medals (even a gold for Jenny). During that time, nobody has been anywhere near the front in the 5000.
I would say that rather than accuse Dibaba of blatant doping (who knows, she could be), it makes more sense to think about it that the 5000m talent of the last 10 years is running the 1500 now. Dibaba, Hassan, Kipyegon all ran ridiculous final 800s today, but I bet that when Tirunesh and Meseret Defar were running 14:1x for 5k they could have won the 1500 too.
This year, rather than the best runners running 5k/10k, they are running 1500 and 5k. Which is why all of a sudden the Americans were so competitive in the 10k but were stomped in the 1500.
As a track fan, I would rather think about it this way than just accuse everybody of doping. With that mentality, why even be a fan?