Visual Studio is not written in C#/WPF, it’s written in a mess of legacy COM and WIN32 code with some C# and SWF.

(Visual Studio for Mac is, of course, an entirely different product and codebase).

I thought it was the other way around? C# on the outside, legacy code connected on the inside? Anyways, I found that confusing. For the rest, visual studio most definitely uses WPF for much of the UI but also the chrome (e.g. see https://softwareengineering.stackexchange.com/questions/7532... and https://www.quora.com/As-of-2015-how-much-of-Microsoft-Visua... and https://blogs.msdn.microsoft.com/visualstudio/2010/02/16/wpf...).

I'm quite sure the IDE is WPF. You can attach a debugger to it and inspect the WPF component tree.

You're both right. Yes, it's WPF, and there are lots of parts written in C#. But the parent is right -- Visual Studio is C++ using COM and Win32 at its core, going back all the way to the Visual Basic IDE that shipped as part of Visual Studio 6. It would be a very difficult port to Mac, Linux, etc. As of this writing there is even no 64-bit version for Windows (although that's just a matter of spending the time and money to do it.)

The brand name has been re-used for other software. Visual Studio for Mac is Xamarin. Visual Studio Code is on the Electron platform.

Surprisingly, the lack of a 64 bit version is a performance choice.

”For a long time now developers have been asking why Visual Studio hasn’t made to switch to 64-bit. Rather than effort or opportunity cost, the primary reason is performance.” https://www.infoq.com/news/2016/01/VS-64-bit

All the new code in Visual Studio for Mac for .NET tooling is being shared with Visual Studio, written in C#.

The new plugin architecture, replacing the COM one, is implemented in .NET.

I thought VS for the Mac was just a rebrand of Xamarin Studio?

Initially yes, but they have been making the Roslyn tooling the same across both IDEs.

Visual Studio for Mac has many new features that weren't present in Xamarin Studio, like .NET Core support, ASP.NET types of workflows. And all new Xamarin.Forms improvements are shared between both IDEs.

Number of users an app has is a horrible way to decide whether to write it in C++ or not, and even if it were a good driver Visual Studio is a $1B business for Microsoft so it’s hardly a little niche app lacking a wide audience.

VS has a lot of things going on, many of the teams to specific pieces of functionality that might seem obscure to many of us. The Shell team isn’t that big from what I recall, so yes VS is a huge business but there are a lot of things going on so it’s not like everyone in devdiv is working on core VS. It is really different from a consumer office app.

Visual Studio 2017 launches a child process with a nodejs server upon start for a servicing stack and also javascript language services.

I agree.

The question is whether there's enough of a market between broad-based professional apps and web apps to justify maintenance of the WPF/.NET stack.

How is the maintenance of WPF/.Net better or worse than the JavaScript flavor of the week and Browsers that are now updated weekly - if not faster?

I can understand a "Is it worth the maintenance?" question...

but I would bet .Net has better longevity and Enterprise support compared to the rapidly evolving landscape of HTML/CSS/Javascript.

Well, at least WPF has stayed in existence, relatively unchanged, for what would be eons in JS framework-years.