$5 per month per account. Same price as an entire Office 365 subscription for an API that can talk to your Office 356 subscription. Interesting idea, but your pricing is insane.
Any time you build a large application, you need to be able to choose the best language and platform for any given part of the system. It doesn't matter if you like Node.JS, sometimes Go is a better solution. It doesn't matter if you like Ruby, sometimes you need a little C++. If you get yourself so caught up in .NET that the entire solution needs to be built with it, you are starting out with a terribly and broken architecture, that either will fall apart in the future when you realize .NET isn't the best tool for a certain part of the system, or severely limit your ability to add new features because you never come to that realization.
If you stick with a pure .NET stack, your app will not only be more expensive to maintain, but also have less functionality. The thing you have to realize is that the benefit of using something other than .NET has more to do with how many lines of code you don't have to write than it has to do with how many lines of code you have to write. It doesn't matter if right out of the gate you may be slower with a new platform, because:
a) that won't last long
b) the amount and quality of open source code on the .NET platform is laughable compared to just about any other platform, and that's a very big deal. Open source moves fast enough to overtake giants like Microsoft, use that to your advantage. Your competitors certainly will be doing it.
So what tool would I recommend instead of .NET? Your brain. There is no silver bullet. Choose the correct platform for every part of your system. Embrace open source. Don't reinvent wheels that don't need to be reinvented. Perhaps there will be parts of the system that make sense to write with .NET, but certainly it won't make sense for all of them. There are tools like mono and Xamarin that will allow you to have a bit more platform flexibility while still using .NET. At the very least, get off of Windows and don't pay money for databases like SQL server when there are free alternatives that will better address your needs. Build your backend with something like microservices so that you are free to use something other than .NET when it makes sense. Force yourself to build a part of the system with something else to validate your architecture works even if it is a small part of the system.
As if reading the documents was the problem. This is just some basic wrappers around an XML reader. Hard part is not reading the documents, it's rendering them.
Yet he did ditch the fully open source approach they had going before with Mono... Xamarin Studio isn't exactly cheap. He certainly prefers making money to running a charity.