Just chiming in to say this is an absolutely viable approach. I just built a project, over about six weeks, using mostly vanilla js and some jquery (the heaviest widget I have is tinymce, which is a necessity unfortunately), and started selling it about two weeks in. I'm up to $650 mrr and the sky's the limit (really, the market is huge and desperately underserved), and not one person has complained about it acting like an old style website. People are just happy it works. And mobile testing was a breeze.
If I tried to build this on a "modern" stack I cannot imagine any benefit, only drawbacks.
So unfortunately the live project has no public-facing component; without going into too much detail it helps certain corporations and larger nonprofits with a kind of compliance. But, I can tell you that I based it very closely on the layouts provided by the ultra lightweight css framework, PureCSS. (This is also my favorite framework!)
One other remark: people are happy when they learn about this. That is now my #1 predictor of success. Everyone I've demoed to has adopted the product within a day (usually during the demo), and tech wise it's no different from what was possible with the web 15 years ago.
If I tried to build this on a "modern" stack I cannot imagine any benefit, only drawbacks.