> it's not always economical. For a normal web app - yeah I guess

What do you define as "normal"? I can't think of anything harder to test than a web app.

Even a seemingly trivial static HTML site with some CSS on it will already have inconsistencies across every browser and device. Even if you fix all of that (unlikely), you still haven't done your WCAG compliance, SEO, etc.

The web is probably the best example case for needing a QA team.