> In doing so they neglect/break basically everything else - SEO, connected martech, analytics, etc etc.
This is exactly how it looked like when Flash was a popular choice for making web interfaces. We've fought a long, hard battle to finally get back to indexable, interoperable, standards-based HTML, CSS, and JS. It was fine for a while, then Angular happened. Fast-forward a decade and we're right back when we started. Amazing.
I'm sure some agencies do a fantastic job (those that think about the bigger 'more than just dev' picture). But on 95% of the sites I'm seeing right now the downsides far outweigh the benefits and it feels like dev for the sake of dev.