[kapowaz]

There may not be any conceptual difference, but there is a very significant difference in how these things you describe are used. Flash, when used badly, becomes a self-contained sandbox for the entire experience of a website. Besides security concerns, this approach is damaging to the overall experience of that site, since developers end up needing to re-implement native controls and functionality (scrollbars, form elements and keyboard navigation to name a few) in order to provide users with affordances they would otherwise take for granted. More often than not, the developer doesn't re-implement these features and so users are left with a sub-optimal experience (even if it is more ‘on brand’, or replete with gratuitous effects and animations). This is to say nothing for the accessibility implications.

Since the advent of smartphones and tablets that can't display Flash content, sites that do this have started to die out in favour of a combination of native applications and true HTML implementations. If the experience they're going for calls for a video or canvas element (3D or otherwise) then great, go for it. But the scope of that part of the experience is implicitly reduced, and so it doesn't result in a sub-par experience across the board (I might not be able to see your fancy WebGL 3D canvas effects, but the links to your ‘contact us’ page still work).

What I'm worried about is that by providing a compatibility path, Flash's imminent demise will be put on hold, and the sub-optimal experiences I described above see a resurgence. And that's not a good thing.

[Recoil42]

>Flash, when used badly, becomes a self-contained sandbox for the entire experience of a website.

This same argument is true for Canvas, SVG, WebGL etc.

[kapowaz]

This is true, but because of completely different toolchains it's very rarely the case that this happens. Flash, at its height, was being used with tools like Flex to build full applications. Adobe provided interface libraries for you to substitute native controls for its own (inferior) versions. Nothing like this exists yet for the tools you mention, and hopefully never will..

[sp332]

Flash, when used badly, becomes a self-contained sandbox for the entire experience of a website.

But if it's rendered as part of the page instead of in a black-box plugin, doesn't that whole problem go away?

[Yaa101]

No, It's about what is inside the Flash that defines your experience with it.

For me the best trade-off is still to use a flash-block and grant the flash running time when I choose to do so.