C# is absolutely supported through the COM interface. Although the built in IDE can only do VBA there is a full library that can be used with C# (how most add-ins are built)

Right, you can automate Excel in C# from outside of Excel or make an add-in, but you can't store code inside an Excel document or edit it from within Excel, or assign a worksheet control handler to directly execute C# code, etc.

thats right, but now you can do all that with JS/Html5. All in all I feel your pain though since i fall into this limitation myself (not a huge VBA fan).