But my problem is similar with static sites: The various templates I’ve bought used Jekyll (Python), Gulp and NPM, it is not feasible to explain newbies how to properly install the dependencies for 3 systems (especially Python, with the 2.7 vs 3.7 difficulty).

Wordpress is inherently unstable on the long term because plugins must be upgraded (anyone who’s had a vulnerability issue knows it). But there is no other decently simple technology.