Mainstream as in, in the majority of people's homes? I can agree with that. Look at 5.1. The majority of homes today at best use a soundbar. So in that sense, 5.1 is a gimmick too. But Atmos isnt going anywhere.
Before 5.1 people had pro-logic home theater systems, so there was no need to overhaul your speaker system, all you needed was the software and a Dolby Digital A/V receiver. First there was Laser Discs, they were Dolby AC-3 encoded (some were DTS encoded), back then, you couldn't have Dolby 5.1 playback unless you owned a laser disc player and some titles on laser disc. Laser Disc players were expensive and laser disc movies were expensive, like $50.00 each, they never went mainstream. Laser Disc players and Laser disc movies eventually ended up as a niche product, only found for the most part, in high-end stores. Dolby 5.1 would have died, but then came DVD which exploded and became the most successful consumer electronics product ever and Dolby 5.1/AC-3 became the standard audio format for DVD.
DVD Players, after the first generation, became widespread and affordable and even offered better picture quality than Laser Disc. Dolby 5.1 became mainstream in the advent of the world wide success of DVD. After DVD, came Blu Ray, Blu Ray is successful, but not as successful as DVD, DVD still sales more than Blu Ray. I know that's hard for some enthusiast to believe or accept, because Blu Ray is so superior to DVD. Dolby Atmos is reliant upon the long term viability of Blu Ray as medium. However, streaming has surpassed to sales of Blu Ray and DVD and threatens to make optical disc obsolete, unfortunately so because Blu Ray is, in my opinion, superior to streaming content. Now Dolby Atmos is piggybacking off of the Blu Ray format while the Blu Ray format is a sinking ship. So how is Dolby Atmos viable? Why would anyone invest in A/V receiver, extra speakers, subwoofers, even to a degree, retrofitting a room for Dolby Atmos when the future doesn't look bright for home Dolby Atmos? Unless you tell me we're going to see 11.2 channels of audio streaming.
I'm telling you, I've been in this hobby for awhile, I've seen formats come and go and some die on the vine. Look, people thought that 3D TV was going to be the next great consumer electronic success, but it failed. I own a 3D TV, I have few 3D Blu Rays, I have yet to experience streamed 3D movies. Also, 3D movies are few and often $10-12 dollars more than the 2D versions, which in my opinion, hurt the viability of 3D TV as a consumer electronics success. I have heard that some TV manufacturers are dropping support for 3D TV, not even included it on their new 4K displays.
I don't think we will ever see Dolby Atmos in homes to the level of 5.1. I don't understand it, most people can't even get two channel right and they want to jump to 7.4, 11.2 etc.