> They are some of the most capable and ambitious pieces of software ever written—and yet it’s commonplace for small teams of developers to toss aside large pieces of built-in browser functionality in favor of doing everything from scratch.

The author highlights something very important here. There's no substantial offline mode for HTTP. The request-response nature of the HTTP protocol has no standard or policy for what to do if the request can't be sent. Browsers have grown to include some ability to operate offline, but it's an afterthought. Firefox's offline mode doesn't do much more than allow for browsing cached pages, which isn't much good if those pages run javascript that needs connectivity. Much of the rebuilding-from-scratch in SPA development is to supplement or extend the browser's functionality to be offline-friendly.

As the author notes SPAs arose at a time when connectivity was much more intermittent. Today, the areas of the world with poor connectivity are of little to no interest to businesses that profit from being on the web. Browsers don't support offline mode better because there's no financial demand for it.

This leaves offline mode as a niche where the's little or no money to be made, so hardly anyone cares any more. It's not possible to serve up targeted ads through real-time auction bidding when the user viewing the page isn't connected.