You'd specifically see it with tiling window managers like i3. I can put any container node of the window tree in tabbed, stacked, tiled horizontal or tiled vertical mode. So, I can have tabs of any mix of programs, I can change the mode to see all tabs at the same time, each tab can have subtabs or windows organized in any way I see fit, I can fullscreen any level, etc.

The handling of those tabs also fit with the rest of the window management, like putting marks on a container so new windows appear at whatever place in the window tree I wish (like having a shell to my left and all windows I'm invoking having them appear organized in an arbitrary manner in a window container to my right), etc.

I can keep going, but if you're not familiar with tiling window managers, I feel like probably none of what I'm saying is making any sense...

I'm familiar with i3, but I feel as if the other features of i3 (configuration via text file in home directory (don't worry, the EBNF is in the header and the examples are annotated!), minimalist presentation, keyboard-first interaction (I still have a printed hotkey chart...)) are likely to stymie adoption even further than Sets, which are simply obscure.

Those other "features" are just conventions that come with being on a *nix OS. They're aren't unique to i3. But regarding adoption, of course power-tools (for power-users) don't get wide adoption. That's just how things are, since the majority doesn't have the time to put into learning how to use such tools.

This thread is about there being a use for a tab-less browser. That doesn't necessitate wide adoption.

(Not that I'm saying that a tab-less browser is of particular use, since one can just ignore tabs in a tabbed-browser.)