The amount of money we make selling to customers using older browsers vastly outweighs the amount of money we spend on maintaining our site to support them. That’s even true for IE7, which only accounts for a seemly pitiful 1.2% of the traffic to our site, but it’s true: we make much more money out of customers using IE7 than we spend supporting their antiquated, broken-down old web browser that they really should replace.
Supporting old browsers is an investment. There might be some return thereof and you even might break-even, but that doesn't mean that this was the best investment you could have made.
I know of one company which managed to increase their revenue by more than 100% just with front-end performance optimizations.
Supporting old versions of IE is generally more work than taking care of basic optimizations.
Another thing is marketing. Virtually no one heard of your product/service. Getting twice as many people on your site will be way more useful than making those 10% with IE8 happy.
Support for legacy browsers should be the last thing on your list. It's the least interesting one. The payoff is tiny (and it even decays over time) and it also makes maintenance more expensive.
That’s great but, if your site has an MVC back-end, and you’re used to working in Visual Studio for both front and back-end, you’re somewhat on your own. There’s no tooling for Visual Studio [...] it’s going to break your workflow.
There also isn't any tooling with JavaScript, SCSS, or Handlebars templates. Well, I really don't see the problem. You can run several IDEs simultaneously. PCs are surprisingly powerful these days and RAM is also dirt cheap.
You’re developing a single page app (I’d say avoid it for multi-page for now)
I'm not really sure why he thinks that it's only usable for SPAs. for example, you can also create a bunch of independent widgets which can communicate via events (see: Nicholas Zakas - Scalable JavaScript Application Architecture). You can also use it for games, for server-sided stuff, or command line tools. You can also embed the VM in other applications and then use Dart for scripting.