Have you tried to implement interaction-rich applications such as calendars or text editors with server-side rendering? You won't be able to recreate the UX an SPA can afford, regardless of revenue.
Widgets require JS, of course. Javascript was born to sprinkle some of it on top of an HTML page, not to make a whole meal out of it. This is what GP is arguing about and I tend to agree. Single Page Apps are not worth the effort in my experience, apart from very niche use cases.
The velocity proposition and fantastic developer experience of writing JS in the backend and frontend and data layer is overrated and misleading. It's everything but.
I don't think anyone would take fault with using a SPA for a text editor. What most people are criticizing is using a SPA for a page that serves mostly non-interactive content. And that is the majority of the web, after all.
On whatever page you're using websockets, you're basically doing what an SPA does. If you click something and the page sends and receives a message via websockets and then updates the DOM, congratulations, you have something that's the same level of complexity as a SPA.
Your original post says that this style of interaction only exists to "fleece VCs". But it seems you're doing that, with an additional added layer of complexity on top (namely the server-rendered pages).
I'm guessing you implement the interactivity with <script> tags or something in your html/template templates, skipping the pain of npm and bundlers. I'm not sure that's the hard part of SPAs, though. Just an annoying one. The hard part is synchronizing the state of the frontend and backend over a communications channel.
Except not really, because one is architecturally simple and relatively easy to grok/hack, and the other is big and complex and built specifically for operating at a massive scale and multi-team environment.
We're talking ~200 combined LOC of idiomatic Go+TS & a couple popular libraries vs. an entire SPA framework and its various DSLs.
State synchronization over a long distance is always going to be a balancing act of performance vs. reliability vs. security. It's never simple and outsourcing those decisions to a framework is not always the correct choice as it may bite you later.
