I disagree with most of the benefits you list

I'm curious to hear your objections.

chief among them “increased security”

Do you deny that specific protection to some attacks is provided through the chosen model of 'immutability' on at least one of the atomic distros?

not to mention half of them are already supported by traditional package managers

Hmm...,:

• atomicity; nope
• reproducibility =/= reproducible builds for some packages (if that's what you meant)
• declarative system configuration; ansible (and any other solution that I've witnessed being mentioned in such discussions) succeed (at best) at convergent system management, while e.g. NixOS does congruent system management by default. Consider taking a look at this page if you're interested in what these are and how they're different. (Spoiler alert) congruent is better and therefore more desirable.
• increased security; security is not limited to chosen model for 'immutability' if at all; as Qubes OS (read: most secure and private desktop OS) doesn't rely on it for its security. So I can understand where you're coming from, but I have yet to see any non-security focused distro that provides the elevated protection against particular attacks that some atomic distros offer by default.
• built-in rollback functionality; sure, this is not exclusive to atomic distros. Perhaps I should have done a better job at making clear that it isn't a feature provided necessarily by atomicity. But, the fact that I listed it at the very end, alludes that it isn't as exclusive and consequential as atomicity is. At this point, however, it has become almost synonymous with atomic distros, while the same can't be said about traditional distros.
• regarding the consequences; I'm unaware of any distro that does those out of the box (barring Pop!_OS with their factory reset). Though, I'd love to be educated on this.

I was genuinely curious so thanks for the rationale.

It has been my pleasure ☺️! I'm also genuinely curious to read your reply to this comment😉.