Making lists is easy, but making those lists accurate and internally consistent is more difficult.
First, you claim that the OS is “read only”, and “immune to malware”, yet you’re also claiming that you can install completely arbitrary executables like Firefox on it?
Those statements are incompatible, unless you deliberately modify Chrome OS in unintended ways to enable it to do things that it was never intended to do. Much like you’ll inevitably be able to do with Windows 10 S. And it’ll make you more vulnerable to malware along the way.
The fact that Windows 10 S is locked down to only apps from the store means that it too is “immune to malware” - at least to the same extent that Chrome OS is. Since you don’t even have the option to install arbitrary applications, you can’t (purposely or inadvertently) run malware.
There are still vectors to get malware - just like Chrome OS - through inevitable browser vulnerabilities, or rogue browser extensions or apps which get past Google’s or Microsoft’s approval processes.
Chrome OS also does not support Android apps quite yet. It was announced over a year ago, but Google hasn’t delivered on it yet:
Meanwhile, the Windows Store supports full desktop-class applications, nearly 1000 of which are currently in the Windows Store. These are all fully-vetted applications which Microsoft has verified to be free of malware.
For example, I’m currently using Slack and Evernote through the Windows Store and it’s a whole lot nicer to do so than using their custom installers and having yet another third-party updater service running and consuming resources in the background. Adobe Photoshop Elements is in the store too with the same benefits. The full version of Office will be there soon too.
It’s early days for Centennial, but with BUILD going on right now having a major focus on Centennial, and tons of developer interest, it shows that while the store might not be there right now, it’s certainly capable of providing a more robust desktop-class experience than a Chromebook without sacrificing performance, stability, or security.