I've come to the opinion that for the vast majority of apps I've built, it could all be built using HTML + CSS (all built server side). I can sprinkle in little bits of interactivity using something like HTMX. And I'll have a website that is very easy to optimise, has phenomenal backwards compatibility, and gets rid of a whole class of issues associated with SPAs.
I often regret in my career not pushing back more on "requirements" that ended up requiring a more complicated app, whereas the customer would have been happier with a simpler solution.
No need for HTMX, HTMZ can get you most of the way there if it is going from simple MPA to slightly more complicated. I used a variation of HTMZ to make a offline-first soccer app I use for myself. I thought I would need to use a front end for the match play page, but, nope, I used Morphdom with HTMZ and I was able to keep the simplicity of templating and a back end.
I often regret in my career not pushing back more on "requirements" that ended up requiring a more complicated app, whereas the customer would have been happier with a simpler solution.