Of the top 20 performances ever in the Men's 10000m, just five came from the pre-EPO testing era. Of the 67 times 27:00 has been broken, 53 of them occurred after the EPO test had become available. In the 5000m, 13:00 has been broken 249 times: 73 before 2000, and 176 since. It seems that your argument relies heavily on a few outliers. Take out Bekele, Gebrselassie and Komen, and the 5000m world record is Eliud Kipchoge's 12:46.53, and suddenly Ritzenhein and Mottram are less than 10 seconds out from the world record, as opposed to nearly 20. Same in the 10000m: take out Gebrselassie, Bekele and Tergat, and your world record falls back to Kemboi's 26:30.
Could I have not made similar arguments 25 years ago, saying that the 800m, 1500m, Mile, and 5000m were all taken apart from 1978 to 1985? And in retrospect, Coe, Cram and Rono are judged as being among the greatest of all time (and Moorcroft as having the race of his life in a period of injury free training in 1982).
I don't see how times have regressed. As I stated above, most of the fastest 5000m and 10000m times have been recorded this decade. In the 1500m, you could argue that 39 sub 3:30s were run before 2000, compared to 33 since, but El Guerrouj alone represents 44.4% of all sub 3:30 performances. Apart from him, 27 sub 3:30s were recorded before 2000, and 15 since. And of those 27 run before 2000, 16 belong to Morceli and Ngeny.
And the marathon and half-marathon record books have been totally revised in the last few years alone.
Is the regressing you're writing about in reference to the absence of many performances approaching the world record at those distances? Brussles will often cancel its Men's 10000m if Bekele isn't up for a bid for the record, because the idea is that it'll be boring for TV and spectators if the record isn't on the line. And in the 5000m, the record is out of reach for most, to the point that any serious attempt needs a pacemaker who can run 7:35 largely on their own, and there aren't many men with that ability who wouldn't want to be in the race themselves.
You're right, many could have taken EPO in the 1990s and gotten away with it. But just like any opportunity to commit a crime, some will take it, some won't.