> Javascript performance on mobile has been doubling roughly every generation
This is a very strong statement. And even if it was true, there still are fundamental limitations to Javascript performance, i.e. you wouldn't expect it to reach the native performance.
It is true in so far as if you run benchmarks on the Nexus 1, the Nexus S, and the Galaxy Nexus, the Sun Spider and V8 benchmarks roughly double between each device. Some of that is improvement in V8, and some of that is faster ARM CPU.
I don't think anyone is claiming JS will reach native performance, or that you're going to write triple-A title games like Battlefield 3 in it. The question becomes, when is it good enough?
At a certain threshold, JS performance will be good enough for a good swath of apps and games on mobile that the deployment benefits and cross-platform nature will outweigh the downsides for many developers.
Just look at JS on the desktop. Performance has gotten good enough that for the vast majority of cases, people do a good chunk of their work within the Web. There's no need to download native apps for reading news paper sites, or social networks, or even light productivity work on your desktop.
In my opinion, too many apps are being written in native code for mobile that don't really deserve to be, it's a temporary transient situation for a few years.
IOS basically is a return the Microsoft era with native apps for everything, and I think, like with Microsoft, the Web will start to turn around in the future and start to steal back marketshare from native apps.
This is a very strong statement. And even if it was true, there still are fundamental limitations to Javascript performance, i.e. you wouldn't expect it to reach the native performance.