The only things I can think of are Flash in YouTube provides far more control over playback for things like downloading, ad delivery etc.

But fundamentally... Street View. I can't see how that will be delivered with high performance by HTML5 any time soon.

It doesn't seem that unlikely, street view is mostly a fancy slideshow.

It does some pretty sweet 3d-aware image manipulation. I wouldn't call that a slide show.

On the client end?

Yes.

WebGL or perhaps fiddling with Canvas. It's only a matter of time.

Street View already works in the WebGL version of Google Maps, with performance no worse than the Flash version.

Then I retract my half-hearted defense that perhaps Google is invested in Flash.

Which is why they're furiously working on MediaSource API (and it's been in Chrome for a while for those who pay attention to their about:flags)