EDIT: There is also different ways of using flash.. the proper way (in my eyes) where you add to the effects of a website... or the improper way (in my eyes) where you make entire flash websites..... if you use the later (which a lot of people do for some reason) then HTML5 (as a part, or the entirety) is a DIRECT competition to Flash.
That's a bit simplistic. Certainly you don't want to make static text pages in Flash if you can help it. But there's nothing wrong with making all-Flash sites if it fits the purpose: high-end multimedia experiences. Look at
http://audiotool.com for example. Front page is HTML, but the main guts of the site, or "app", are entirely Flash. You could not do that site in HTML5. Javascript doesn't have the horsepower or the APIs necessary to manipulate byte arrays at a low level in that way.
The blind hatred for Flash around here is a bit confusing. When HTML5 starts being used for heavy animations everywhere, people will curse it just as much. (I've been forced to make such things in HTML5 before -- it's not pretty. Things start to buckle when you've got complex, fullscreen animations going on.) As others have said, 90% of the time it is the people making the content, NOT the tool. Or people choosing the wrong tool for the job. There are places where HTML5 makes sense, and places where Flash makes sense. It's not a binary choice of existence. Both can co-exist. Adobe certainly thinks so. A lot of these anti-Flash cats sound like the guys who internet-rage in a Playstation vs. Xbox war. People who actually create content (and I am occasionally one of those) use whatever tool we think is best for the job. Sometimes we're forced to use what the client wants.
I'm not dissing "HTML5"; it's about time HTML started catching up with Flash. But Flash is so far ahead from a programming language & API point-of-view, HTML/Javascript always be playing catch-up. Especially since Adobe is not bound to a standards compliance body that is constantly in-fighting (go read up on the history of Javascript and realize it hasn't had a real update to the language in 10 YEARS because of corporate bickering). So I guess yes, I am dissing Javascript. It's stuck in the 90s. Terrible language. But I'm not dissing the concept around HTML5; that of adding APIs to allow for multimedia integration into web pages. It's just the execution of that concept that is a bit wonky right now. Unfortunately, the Javascript standards body is still as incapable as it ever was, so I don't see much improvement there. Web browsers can only strap so many rockets and nitrous packs on an '83 Honda before they run out of room.
Adobe is currently beta-testing a version of Flash with full OpenGL 2D and 3D hardware acceleration. Playstation 2+ level graphics in a web browser at 60fps. You can't do that kind of thing with Javascript and HTML. And you shouldn't!! It's also receiving full multi-threading. Flash is sadly leaving Javascript in the dust. But as I said, there's room for both technologies. So chill out guys. Please.
And as a final thought, I think most of this internet rage I see is coming from the fact that most sites are laden with ads. Ads all competing with each other for your CPU cycles. Page loading slowed down by AJAX calls that load in the ads from third-party servers and wait to load the page content until the ads are received. I know, this sucks!! But replacing Flash with HTML5 ads won't change that situation at all! But this is a basic problem of the web. The fight between putting sites behind paywalls and "free" content was decided many years ago (aside from NYT). Free content won out, as "free" everything usually does. But "free" in a marketplace is usually a devil's deal. You're going to pay for that content one way or another. So instead of subscribing to sites, the sites are afflicted with ads. But sites have to make money. If users want to change this, then perhaps people should band together and figure out a different business model. Blocking ads isn't the answer; that's equivalent to pirating music and movies because you think the price is too high. I would welcome a microtransaction kind of deal where a pageview is one cent or something, deducted from your account, but no ads on the site. That's effectively what we're going to with music and movies...a global subscription model, but I prefer something more granular for the web. But unless customers band together to enact this kind of sweeping change, the status quo will stay.