The idea of a Linux version does seem ridiculous like you said, but I'm not sure lack of 64-bit is a good example to illustrate it. They already migrated much of VS2008 to .NET when they did 2010, which to me seemed like a herculean feat, for what was quite questionable end-user gain (and I'm being quite generous with that assessment), and .NET should've made architecture-independent easier. Lack of 64-bit just seems to be due to the cost not justifying the benefit rather than the feat itself being so herculean per se.

The main problem there is that it runs like a dog and thrashes when you have big solutions and get near to 2GB of RAM usage. It's pretty silly on workstations that have 16-32 GB of total memory available. When they first justified the decision not to move to 64-bit a decade ago, it was more defensible, but time has moved on hard, and they've been fighting that decision, moving bits and pieces out of process ever since.

This should be possible, as the Mac version is pretty much a coat of paint and a blessed set of plugins laid on top of MonoDevelop.

If I had to do serious dotnet work on Linux, I'd probably just use JetBrains Rider instead though.