Earlier quoted context omitted.
They really weren't a thing, but they also weren't unheard of. Personally speaking, I started working on a complex JavaScript application (web-based ERP/CRM system) in 2000. While it wasn't an SPA, we designed our pages so you didn't have to leave them to do what you needed to do. For example, a user might stay on one page for a few hours because they're speccing out a project. Updates were often committed live using…
Should Microsoft get credit for inventing Ajax and having the first real world functioning example of it?
That's what Java applets and plugins were for.
I'm also pretty sure they weren't the first to fetch data from the server without reloading, because people were doing all sorts of novel things in the mid-90s with plugins.
The reason it didn't really take off back then wasn't because of vision. It was because there was so little you could do with JavaScript. The browsers were still implementing the basic features and a lot of page/component manipulation was very broken. Applets and plug-ins were so much more capable.