I don't personally care much for DSP stuff when it comes to music.***
Lots of people have dumb, unfounded prejudices about all manner of things.
Estimate $20k vs. (I presume) $100k-$200k Atmos system? That would be interesting.
It seems like the top dog Atmos system with upfiring speakers available today would use KEF R700s across the front*, their new R50 Atmos module on top, and perhaps their R100 stand mount or R800ds bidirectional out-of-phase surround in back. Drive those speakers with sensible electronics - smart people know they don't need to list off exotic brand names; with rare exceptions that incorporate cutting-edge room correction - the Datasat, Trinnov, Anthem Statement, etc. boxes - none of the exotic electronics out today are even as good as a modern AVR, let alone better - and prices will likely be competitive with an LCS setup.
How might Atmos system decode regular 2-ch? Do they even care about this or do they address only Atmos encoded program?
See supra. They have a new upmixer, called Dolby Surround, that works with 2.0 - 7.1 channel source material.
Up to now, does anyone prefer any multi-channel decoding over their most favorite 2-ch system?
I usually do. Not always but usually. Most of my 2-channel listening is expanded via Dolby Pro Logic II.
I presume several tens of thousands for audiophile quality and considerable real estate in the room.
Depends on how intelligent one is about setting up the system in the room. If one is a renter, or an uncreative and cheap homeowner, options are admittedly more limited. For the multichannel music system in our general-purpose living room, I designed and commissioned a bespoke mantel over the fireplace for that integrates the center channel. It houses a a Pioneer EX, i.e. baby TAD, in-wall unit. Left and right mains (also Pio EX, with Aurasound woofers in the flanking subs) are freestanding. Side-surrounds (Pio EX again, but with a single 8" TAD woofer above the concentric driver instead of the twin 7" woofers flanking the concentric driver the LCR mains) are in the front walls, slightly ahead of the listening position. Rear-surrounds (KEF R800ds) are on the rear wall, above some leaning bookcases. Two subs are "flanking subs" under the mains, a third is in a corner, and a fourth is along the opposite side-wall. While my room isn't equipped with height speakers, it is prewired for a "7.multisubs.4" setup, for height modules atop the front mains and rear surrounds. Total system outlay was well under $25k, including all of the bespoke work and the non-audio stuff (TV mounting, HDbaseT hardware for TV, etc.). True, it would be higher if I insisted on stupid "audiophile" electronics instead of being smart enough to use commodity parts that work well. But I'm under no delusion that anything "audiophile" is of any value at all. In fact, "audiophile" is generally a code-word for "scam" today.
So basically a well-integrated multichannel system takes up no more room floor-space than a competently-executed 2-channel system. (Multisubs are required for high fidelity upper bass either way, so I'm not counting those as a distinction.) Less, perhaps, considering I'm not a deaf buffoon who wastes space on pointless things like "cable risers."