I would agree with the sentiment that people who think JS (or HTML5) is some panacea for Flash are idiots. Flash was hated primarily because it was TOO popular - sites abused the fuck out of it and multi tabbed browsers sagged under the weight of so many running instances. If JS is abused the same way the performance would be just as bad.
JS is often considered the problem, not the solution to web development. This is why coffeescript, typescript et al exit. Plus a raft of JS libraries like jquery, backbone, underscore, phantom, handlebars etc. to hide the differences or provide basic niceties that JS lacks. Plus the likes of dart, emscripten, GWT and so on which bury JS completely and spit out compiled JS. Plus the recognition from browsers that JS performance sucks and the optimization paths they've implemented (e.g. asm.js). That said, we're almost in a place where 95% of the use cases for Flash are probably achievable in JS. Personally I wish browsers would adopt PNaCl or something similar so code can be compiled and run at near native speeds - skip JS as an intermediate format when it doesn't make sense and just let sites ship bitcode.