The problem with your theory is that all of those factors were also present to an even greater degree in the 1970s but guys were running a lot faster than in the 1990s. Just out of curiosity I compared 5 years of CIF finals of 1973-1977 with 1993-1997. I started with 1973 because it was the earliest year I have times for. The difference really surprised me.
Here are the top 5 finishing times for the respective years. And remember, the 1970s guys were running 2 miles so that makes the time gap between the 2 periods even greater than shown.
Top 5 finishers 1973 - 1977 (2 mile times)
1973: Williams – 8:54, Taylor – 9:00, Davis – 9:02, Beck – 9:02, MacMitchell – 9:07
1974: Kimball – 8:46, Hulst – 8:50, Schankel – 8:56, Hart – 8:57, Pfeffer – 8:59 (10th place was 9:08)
1975: Hulst – 8:44, Serna – 8:45, Kissin – 8:56, Kingery – 8:57, Aguirre – 9:00
1976: Hulst – 8:57, McConnell – 9:00, Spilsbury – 9:01, Holmes – 9:02, Schultz – 9:03
1977: Assumma – 8:52, Berry – 8:53, Elieff – 8:57, Alvarez – 8:56, Ortiz – 8:57
Top 5 finishers 1993 - 1997 (3200 times)
1993: Farley – 9:04, Keflezighi – 9:06, Casillas – 9:13, Martinez – 9:13, Love – 9:17
1994: Keflezighi – 8:58, Arce – 9:03, Farley – 9:03, Knutson – 9:05, Martinez – 9:06
1995: Balkman – 9:05, McLarty – 9:07, McQuitty – 9:10, Sanchez – 9:11, Arce – 9:13
1996: Gillen – 9:03, Bernaldo – 9:04, Chavira – 9:10, Sanchez – 9:11, Maestretti – 9:14
1997 : Festa – 9:06, Rodriquez – 9 :07, Creasey - 9:15, Buchanan -9:15, Nelson 9:16
I think a lot of it comes down to expectations. In a highly competitive environment everyone runs faster. If you know there are guys in your state or section running sub-9 then you are more likely to train and prepare accordingly. Would Meb have run faster in a different era? Almost certainly. The drawback, when it comes to HS running, is that some of those 1970s guys like Kimball were fried at the end of their HS career from overtraining and over-racing. It makes you wonder what Kimball, Hulst, et al. could have run with contemporary equipment and training methods. How fast could Jeff Nelson, who ran 8:36 for 2 miles in 1979, have run with super shoes and today’s training methods?