Rather than an either/or between purist 2-channel and a full-on Atmos setup, 5.1 is a satisfying halfway house. I can say from experience Atmos folds down into 5.1 very well, and conversely, a lot of 2-channel content upmixes/dynamically matrixes into 5.1. I’ll go a little further: sometimes the 2-channel content sounds better in 5.1 than the Atmos mix.
The Atmos fold-down into 5.1 is no accident; Atmos is based on 5.1 as the starting point, with additional spatial information tacked onto other "stems" as they pop in and out of the mix. So 5.1 playback doesn’t actually lose any information; overhead content is simply re-mapped into the 5.1 plane of sound.
But down-mixing Atmos content into 2-channel goes too far; too much is lost, and a separate 2-channel mix is usually recommended.
So a system with purist 2-channel, combined with 5.1 in the same room with a bit of extra switching, lets you enjoy all the dominant formats: 2-channel music, movie and TV soundtracks, and most of what Atmos offers.
I personally found the Atmos mixes kind of variable: the "Rocket Man" remix is stunning, no question about it, but others were in poor taste, or just flat-sounding, with nothing but a splash of reverb in the rear and sides.
Being able to switch on-the-fly, in the same room, between purist 2-channel, upmixed/DTS enhanced 5.1, or Atmos in 5.1 let me decide which mix sounds best. And the answer was they are all over the place, between option A, B, or C.
In theory, yes, Atmos should always sound the most modern, with all the studio tech that’s available today. But ... with the heavy compression we hear on a lot of modern "remastered" recordings, the actual musical experience might not be as satisfying as a plain old 2-channel CD made in 1990. An awful lot comes down to the taste (or lack of it) of the re-mastering engineer.