It was never quite for me apart from, like, SGC2C (mostly before there was ever an AS), ATHF before it went to shit, and The Brak Show before it went to shit
I also blame its expansion for CN spending most of the time since 2001-02 or so being a demographic-pandering mess or embracing AS-esque hideousness
So yeah, why celebrate Adult Swim? I feel like it mostly embraced/codified the nihilistic, sophomoric sense of humor plaguing "adult comedy" animation, and I admit that it is weirdly disturbing to see Toonami but in HD and TOM is allowed to swear now.
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Comedy Central also had a late night cartoon block. It was pretty good, as I recall.
(Most likely I did know about this in 2012 and managed to forget in the interim.)