Alan Moore actually originally wanted to use a bunch of characters that DC had just bought, in order for the characters to be recognized to the readers before he revealed their flaws and/or killed them off.
Needless to say, the DC editor that read his proposal did not believe DC would be willing to have characters they'd just spent good money on killed off, or (much worse - death does not need to be permanent in these universes after all -), morally compromised in a way that readers would not forget. So Moore was convinced to go back and find a way to make it work with new characters.
> "All but the last issue feature supplemental fictional documents that add to the series' backstory ... Structured as a nonlinear narrative, the story skips through space, time and plot."
Similar could be said for the "Superhero without superpowers" plot device, and "Heroes with fairly regular human problems" plot device. The plot devices were old and well tested by the time they were used in The Watchmen.
On the other hand, the idea of using a limited series outside of the normal (cannon) universe to both introduce new heroes and kill them off in the same book/series was a deviation from the usual comics industry methods (for maximizing profits). Of course, DC eventually did a follow-on prequel "Before Watchmen" in 2012 after the release of the movie adaptation of the original in 2009. The price of success in comics is typically repeated dilution.
Permanently killing heroes became normalized in 1988 with "A Death In The Family" when they killed the second Robin in Batman (and started the "Replaceable Robin" and/or "Replaceable Sidekick" trope).
I've never seen the Watchmen as an analogy to the comics industry. After all, it was from one of the big two publishers. To me, the Watchmen was mostly just a product of its time; at the height of the seemingly endless threat of the cold war when everyone wanted the cold war to end. The trope of "endless threat resolved with substantial losses and significant change" was fairly prevalent in stories from the mid 80's. To some degree, the trope is still reused in current works, but to a far lesser degree. The toughest thing to grasp is what the mid-80's were like, and unless you're in your mid-40's or older, it will be tough for most to imagine. If I didn't know what the time was like, I doubt I'd consider the Watchmen to be such a fantastic story.