Do non-techies have a choice, though? Given the lack of theming in many modern apps, its not like users have a choice between flat design and the designs of the 90s and 2000s. What about sticking to old versions? I can only use old versions of software for so long before it becomes infeasible for many reasons (security and interoperability being the biggest reasons), and even on Linux where users have a great deal of control, not all GUI software is themable.

It's not just programmers who complain about modern UI/UX trends. The Nielsen Norman Group, a UI/UX consulting group that was founded by HCI legends Don Norman and Jakob Nielsen, have written articles against the overuse of flat design (https://www.nngroup.com/articles/flat-design/).