I love the fact that there's an OS-in-OS. It makes writing cross platform applications with easy distribution and access to rich APIs very easy.
Of course if you use a fancy webmail, Google docks or even something like Facebook it makes sense to have a full blown webapp but I personally seldom use these. But the thing is whether or not you need those feature they still bloat your web browser because you have to have it. How many websites ought to require JS to function properly out there? How many do?
Have you tried browsing the new Reddit interface on a low power smartphone? So much richness that it's barely usable. All that to display a list of titles or text comments. Or how about Discord which manages to lag in my browser despite being effectively a chat application.
Sometimes less is more. We've made it normal for people to use software that by default automatically runs a turing complete programming language for any random website you browse. If we had a tiny little bit of friction for going from website to webapp developpers and managers would think twice before cramming "rich" features and a thousand ad scripts everywhere.
You're right that they're not completely read-only documents but the interactive functionality is limited enough that it doesn't really qualify as an app. You don't need a client-side turing complete language to implement it, just forms. It's more akin to filling some paper form than interacting with Visual Studio if you see what I mean.
Most of the web is still designed around displaying text. Even SPAs that require javascript to display text, for all the faults and valid criticisms that model entails, is still no less of a document in terms of its intended function than a bare HTML page.