HTML5 video and Apple's refusal to allow Adobe Flash Player on iOS didn't kill Flash, it just changed the way people use it. Flash still offers so many features unavailable in HTML5 -- such as a variety of streaming modes, multicasting, DRM and closed captioning -- that it's still widely used, both on mobile through AIR apps and on the desktop through the browser plugin.
There are three categories in the desktop market as shown in Table 1. These include legacy browsers that don't support HTML5, HTML5-compatible browsers with the Flash Player installed, and HTML5-compatible browsers without Flash installed.
Legacy browsers, no HTML5 support
This group primarily includes those still using pre-9 versions of Internet Explorer. As of Dec. 1, users of IE 6-8 included about 32% of all users as measured by NetMarketShare, with a few percentage points of other legacy browsers also in this category.
An HTML5-only approach without fallback to Flash would not reach any of these viewers. As you can see in Table 1, Flash does a nice job supporting these viewers, enabling both single-file and adaptive streaming and all the advanced features. Or, if the website uses an HLS compatible player like the JW Player, HLS will also deliver adaptive streaming, DRM and closed captions.
Table 1. Desktop markets and video-related features.
For small websites that only want to send a single video stream to their viewers, this is an easy group to serve: Just deploy an HTML5 player with Flash fallback and you can serve virtually all desktop and mobile clients. The Flash Player also enables access to all the other listed advanced features, including adaptive streaming via Flash or HLS.
If you decided to exclusively use HTML5 delivery rather than Flash, you'd have to supply two codec flavors, WebM and H.264. You'd also lose all the advanced features and compatibility with the legacy browsers in the first group. Definitely not a good move.
HTML5-Compatible Browser -- No Flash Player installed
There aren't many of these users on Windows, but for them the self-inflicted absence of Flash limits the experience to a relatively low level of video: Single stream only with no live video or any advanced features. Because Safari has many relevant features built in, Mac users can access adaptive, live and all advanced features save peer-to-peer and multicast via HLS.