I don't presume to speak for Casey, but here's my 2c.
True, in C#, it's one line to read a file. It's also one line in C once the function has been written, and you only have to write that function once. What has the C programmer gained? They will have learned more about what is going on under the cover.
The overwhelming trend in game development is to
alternate hard and soft layers. High-level languages have been a standard game development tool since at least the days of the Infocom Z-machine (probably one of the first "game engines", if not the first).
AGI/SCI, INF, QuakeC, Edith... there's a long and noble history of high-level languages there. The main change in the modern era is that off-the-shelf scripting languages like Lua (and, yes, C#) are a realistic possibility now.
While Handmade Hero probably won't have a full soft layer (because that's not really the point of the project), what Casey has done so far
is the hard layer. As he rightly pointed out int he video, we haven't actually done any game programming yet; the platform API is, in a deep sense, the virtual machine on which the game will be written.
Do I think Bastion is an underperforming game? No, not at all. But don't forget that a lot of the performance is in the C# runtime and XNA. They are what allow you to deploy the same game to the XBox and to Windows Phone.
If you have an idea for a game and want to get it out there as quickly as possible, then by all means knock yourself out. That's a worthy thing; more decent working games in the world is great and I'm all for it. I'll happily buy it if it's the sort of game I like. But that's not what we're doing
here. This game is handmade.