Quite frankly, I don't remember a really relevant story which focused on this "divergent timeline where superhero exist and the different evolution of this world" thing. Not in the way Kurt Busiek did with Marvels or similar stories in the Marvel Universe. The US where Superman appears in Man of Steel if not that different from the real US in the 1980s, even if in that world the JSA HAD existed in the 1930s.
There have been vague hints, completely irrelevant in the greater scheme, and some of them were also out-of-continuity stories like All-Star Superman. In general, DC stories have never been about a divergent reality where society evolved in a different way because of superpowers. Generally speaking, they are just the adventures of superpowered beings in a world which resembles ours. If they wanted to do something along the lines of the alternate reality, I'd say they could and maybe this new timeline is a good start for that.
Well. No. I could also add that at one point they brutally eliminated important characters like Batgirl (who would probably be in a wheelchair if not for nostalgic fans or writers). But the list could really go on. I mean, basically the entirety of what had been introduced in the Silver Age started getting removed when Dennis O'Neil came on board in the 1970s.Are those all you got?
Supporting the theory that Batman didn't have as many elements removed as Superman is debatable to say the least. In the mid80s Batman was completely different character from the one who had been in the stories 20 years before. Stories like Dark Knight Returns just focus on some basic elements, with zero references to Silver Age silliness, while others like Year One focus on elements which hadn't been really explored up to then (like Gordon) or radically change others (Catwoman as a prostitute).
Removing elements or respecting the order these superheroes appeared in real life, or even their original conception, is relatively unimportant as long as you know what you are doing with them. Batman has simply been in better stories than Superman when the time for making him evolve was the right one. Superman was more or less left at the standing post.
Ok. Let me rephrase that. There are some dumb, silly elements which couldn't be reintroduced the way they were because in that regard modern superhero comic books don't allow that, even if they may be dumb as far as other aspects are concerned.Literally everything about them is still naive and simplistic.
Maybe my point isn't clear. As far as I am concerned, they could have a JSA-less earth or even a Jay Garrick-less one. I don't care. You can do good things with both. And potentially you can have an excellent DCU without Superman being the first hero. There are narrative justifications for him being the first hero and there are narrative justifications for him NOT being the first hero. But nothing is objective in that regard. Heck, you could also say that by starting their new timeline with Krypton exploding in 1938 (if THAT is what they are going for) is, in fact, a way to pay homage to Action Comics 1. There is no right answer nor anything that "has" to be. It's just about tastes and the way they deal with these elements in proper stories.Neither is the meta commentary on Jay Garrick coming before Barry Allen, and therefore the JSA needs to be the inspiration of the JLA in-universe the way it was in the real world.
But you also said the in-story explanation that reflected reality (Barry patterning himself off of Jay, whom he read in a comic book and Establishment the Earth-1/2 divide) was silly, so that doesn't seem very coherent.