Yeah, but Kipsang and Cheruiyot have still both run very well recently. Kipsang was 4th last year, and ran 3:30 at altitude a couple years ago. Cheruiyot was 8th in 3:30 just after turning 19. Ethiopians and Kenyans were historically crushing the 5k last year for the entire season, but they got crushed at Worlds.
My contention is more that if shoes were entirely responsible for the increase in times we've seen, then Kenya would still be dominating the 1500, and Ethiopia would still be dominating the 5k, but they'd all be running 3-4s/mile faster than they used to. But that's not what's happened. Instead, we've seen America/Europe/Australia become more competitive, even as times have gotten significantly faster/fields have gotten much deeper. What that signifies to me is that non-African countries are developing more talent better, for a variety of reasons. I like what you said about Jakob proving that Europeans could compete on the circuit day in and day out, which I think is probably true, and he won Olympic Gold at age 20, while being known for not having a good kick. Kerr and Wightman adapted to be able to run 3:29, and both won for it. Fields are probably so much deeper because there are way more people training to run fast, instead of 75% of them training to run 3:35 with a huge kick.
In the US, we've seen a huge mindset shift at the HS level with Newbury Park's insane success. Everyone thought they were a school of generational talents, but by the time L&L were seniors, people had begun to catch up with them. The whole "4:20 isn't fast" thing made a lot of people mad, but 5 years later most people can't name all the HSers that went sub-4 last year. Hocker seemed like the second coming of Centro in 2021 after he ran 3:50i, won USAs, and ran 3:31 for 6th at the Olympics. Centro got bronze with a 3:34 PR in 2011, only 3s slower than Hocker. Do we really think if WCs were a 3:30 race instead of a 3:36 race, Centro couldn't have run 3:32-33? That's much more in line with my 2s/mile prediction.
This trend hasn't really continued in the steeple or the 10k, for whatever that's worth. Fisher is basically where Rupp was at his peak. He doesn't have a medal, but he's been 4th/5th, and he's come 3rd in the DL 3k/5k (and been more competitive on the circuit in general). His 26:33 is 11s faster than Rupp's 26:44, which is much more in line with 2s/mile. That's a sample size of 1 though. Since we're comparing to Rupp, though, I think it's fun to note that Nico Young just ran 12:57 at age 21. Rupp set his first AR during his senior year of college, a 13:18i 5k. Rupp had the altitude house, but Nico lives at like 7k elevation, trains with world class athletes, and has probably the best 5k coach in the country. And Nico's 12:57 was run at BU with people to race to the line. Rupp's 800/5000 PRs are now slower than Nico's. Rupp was clearly the best at 10k, so I hope Nico runs one soon. It'll be interesting comparing him, Rupp, and Fisher. If they can run 26:20, maybe the shoes are worth 4s/mile.