I'm genuinely curious why you think utilization of the more abundant resource is a bad idea.

Personally I've always wondered why the taskbar and toolbars didn't switch to the side once resolutions got to and especially beyond 1080p

I saw an ad for Opera on YouTube that argued why Opera is better because it lets you organize tabs in groups.

Somehow tab management is the killer feature that will change browsers?

Vertical tabs are working for LibreOffice's menus, which have lots of tabs, and are now getting converted.

Vertical tabs work so well because the width of a screen is bigger than height.