Browsers weren't written in a day. Technically speaking, Mozilla Firefox is a ship of Theseus going back to the release of Netscape in 1994. Did browser and internet infrastructure developers in the early 90s understand that these things would become rich application platforms? Looking at the history of HTTP, it's clear that they expected some concept of "application" to be delivered through the browser. While there's certainly a chance at least a few of them foresaw the full scope of what that would mean (it's not like X11 remoting wasn't a thing), I don't think most of the people involved were thinking much past 10 years (The Distant Future, the Year 2000).
JavaScript was apocryphally "invented in 10 days", it came as an attempt to create competitive advantage, not to create a global standard. The first JavaScript came a year (1995) after the first Netscape, but the first major JS-heavy application didn't come for another 13 years (Google Maps, 2008).
There were a lot of JS heavy web applications prior to Google Maps. From around the release of IE5 at the end of 2000 in particular through the long tenure of IE6. Having worked on some JS heavy applications around that time. It was also much harder as you had a relatively wide variety of browsers and versions. Since people on dialup were far less likely to update their browsers regularly (or at all beyond what came on an ISP or AOL CD.
Of course, the efforts for larger dev teams, optimizations and bundling were far less popular before then. Can't tell you how many poorly written sites/apps carried who knows how many versions/copies of JQuery for example. It was really bad for a while.
Now, at least there are more paying some attention to it. There's still some relatively large bundles that are easy to get overloaded. I mean as soon as you load any charting or graphing library it blows out everything else. Of course this is offset between bandwidth and compute advancements as well.
There was a popular developer site around 1998-2000 or so called 15seconds.com as that was the average point at which users would start dropping off from a load. Now that's measured at around a second or two.
Yeah, I worked on some of them, too. They were even in mapping. I had written one of the first, dynamic, 2D drawing libraries for JavaScript, long before Canvas2D, but it was hidden in proprietary consultoware.
The significance of Google Maps was that A) it had all the parts that we would recognize today as a JS single-page app, B) it had no alternative interface that people could opt to use instead (diluting the usage of that particular implementation versus the product as a whole), C) it had broad appeal and adoption, and D) it was significantly better than competitors specifically because of the "SPA-ness" of the app.
Google Maps had the features and penetration necessary to change the public perception at large of what could be done with browser-based apps.
No it was not. There were many, but for me the most significant early single page app was Outlook web access.
A colleague of mine went diving through the JavaScript source and found a reference to an ActiveX component called XMLHttpRequest. We realised it was pretty useful and ended up using it to build an SPA that approximated a spreadsheet for global logistics planning. It worked very for 2003 standards.
No, certainly not the first in aggregate to use JS heavily. Actually, the browser-based Outlook client was the first to have all the parts that we would now consider to be essential for JS Single Page Apps (because Microsoft has to invent AJAX first). But Google Maps was definitely the first to have a major impact and start changing the public perception of what could be done with browser-based apps.
I consider Google Maps to be the first well-adopted, no-traditional-alternative app to be what we recognize today as a JS SPA. Gmail had a pure HTTP mode, and otherwise was not interesting to people who were happy with their current email. Outlook wasn't really used by that many people, not at the scale that Google Maps was Google Maps has broad appeal and was significantly better because of it's SPA-ness to change how people thought about browser-based apps.
JavaScript was apocryphally "invented in 10 days", it came as an attempt to create competitive advantage, not to create a global standard. The first JavaScript came a year (1995) after the first Netscape, but the first major JS-heavy application didn't come for another 13 years (Google Maps, 2008).