As an aside, we shouldn't even accept your first assumption there, because it's demonstrably not true. There are all kinds of thing that tools like Flash (and Java, and Silverlight, and even ActiveX controls) have previously been useful for that HTML5 and JS can't yet replace.

But your main point is a very good one. There are way too many people in the industry right now complaining about those older technologies who think that the world somehow magically owes them updates to 10-20 years of past work, just because their bleeding edge browser-native tools have finally reached the point in the very recent past when they can do some of the same work to a useful degree.

There are also way too many people moaning about how these plug-ins are such horrendous security problems while ignoring the facts that (a) every major browser also has a less than stellar track record on security issues, with Firefox and Chrome patching numerous vulnerabilities in updates every few weeks, and (b) moving the more advanced, and often hardware-accelerated, functionality from plug-ins to native browser capabilities isn't really reducing the area of the attack surface, it's just reshaping it. Given these facts, there is little reason to assume that browsers will inherently be more secure just because plug-ins were forced out in favour of native technologies. As others have been pointing out today, the reverse might actually be true, because at least the plug-ins have a degree of isolation and click-to-play safeguards are now widespread, while the trend with browsers (again, particularly Firefox and Chrome) is to remove obvious settings that would allow users to turn off certain functions.