[wwweston]

In addition to the possibilities you listed, it might mean:

* most sites aren't SPAs, and progressive enhancement of a document with light interactivity via jQuery is a development model that's a good fit for those sites

* progressive enhancement of a document is an easier development model to move into from having started to author HTML & CSS than many of the SPA frameworks (note that this is also true of PHP), and many training materials that bring people into web authoring use js+jQuery as the next step

* many sites aren't rewriting their codebases particularly frequently, and anything used there remains relevant longer.