After reading it, I'm still left asking why browsers can't do this for the user on mobile as well. User preferences should be respected by default and not require an opt-in step from the webmaster of all parties.

I tried using a bunch of zoom on my most frequented sites and they mostly worked just fine. At my day job everything is tested to work at 200% zoom as a baseline.

I really don't think we should bend over backwards to cater to accessibility offenders such as LinkedIn.

Most any site should work with zoom. This is about the scale of the text separate from the level of the zoom. The latter breaks a lot of sites because many common layouts assume the layout space for the text will always grow along with the text, as seen in zoom.

They can, and they do. Opera does great zoom + text reflow since ≈2010 and counting.