If this happens then the web as a platform will lose. No more Javascript everywhere, forget about all the browser improvements of the last years: we're going Java!
The ecosystem for building apps using Javascript remains strong (Cordova, Ionic, React Native, Titanium, etc). I don't see what bearing this would have on that.
I think you are overdramatizing here. There are always new things building on the web as a platform. I don't really think people and especially developers cared enough about ChromeOS. If anything, Google has just shown more reason not to invest in its platforms early, or even a few years into the cycle unless they have huge commercial traction (i.e. Android).
The browser is a pretty terrible platform for most things to be honest. I say this as someone who is using it to develop their startup as a web app and as someone who has been doing web apps since there wasn't even such a term. When people at work used to tell me how amazing the browser as as a platform, I'd ask them to do simple things that the browser sucks at vs. other platforms. For example, I'd ask people to simply to do various kinds of positioning tasks, alignment, centering, etc. It's amazing how hard at various points in the history of the browser, css, js, etc. it's been to do these seemingly easy tasks. The lengths one has to go to just to get acceptable performance in many scenarios is shocking.
The browser is nice in many ways, but sometimes the ease of printing to the screen is a curse. I am reminded of people writing 20 line BASIC programs way back when and thinking that made them master programmers. The instant gratification and pushing aside of various concerns is great, until you hit those road blocks that are crippling for many types of apps. Somewhere there are old embedded dev colleagues of mine wondering why their new powerful computer is practically on its knees with a bunch of tabs open showing improperly aligned pictures of cats. The browser is great, easy for many things, but that does not always make something good and worth using for every task, or even most tasks. Sadly, a lot of what I see in the browser is cobbled together nonsense that does very little and reminds me of the machine that goes bing.
I know it's an unpopular belief here, but the browser is honestly built on a rotten core from rendering to security to data transport/format. Hindsight is always easier, but the thing is we do have it, and yet we continue to invest in it like there is never going to be another alternative.
I could go on, but suffice to say I wonder sometimes if continuing to build on the browser, and even the web itself is really worth the pain. I wish we'd take a step back and look at the internet itself, and if not that, than at least building other things on the internet like in earlier days. Sometimes I find myself wishing for simple text based menus or something like gopher because at least you found stuff, got things done, and they would be even faster and more efficient now that many of us have decent connections.
if you watch D conference with Bill Gates and Steve Jobs they both say that the browser will take a (giant) step back in favor of rich clients. It only makes sense, stuffing everything into the browser is a terrible proposition on its own for various reasons, but the development tools are very bad too, so no real gain.
You're fine with being to write the same app in various languages? You're fine in having a natural tendency to a monopoly in the OS landscape (because of costs)?
Either you're writing apps you hybrid technologies (like Cordova or electron or the like) or you're already doing that (objc, java, C# to target android/ios/osx/windows)
IF this change made all Chrome extensions go away (which it won't), it would have a pretty minimal impact on the current development landscape.
