Re: Proof in the pudding
For things that serve for more than about 5 years; plan on periodic refactoring or hosting on obsolescent platforms with attendant risks.
Micro Focus Enterprise Server for .NET began development nearly 12 years ago, with an initial release around a year after that. It's a major .NET application - a distributed application server that emulates mainframe CICS and JES. Around 5500 source code files, in hundreds of projects. I won't bother counting SLOC.
We've never been forced to refactor due to .NET Framework changes, or to "host on obsolescent platforms". The .NET Framework is still supported and still receiving updates. ES for .NET runs on all supported Windows versions. (And under Azure too, as a set of worker roles. Or, of course, in VMs.)
You may not like .NET. That's fine. And Microsoft have certainly abandoned some .NET technologies, such as Silverlight, while letting others such as WCF languish after they reached a certain point. It's also true that Microsoft would prefer applications move to .NET Core, and let them abandon features of Framework they're not planning to port to Core.
But it's simply not true that all non-trivial, long-lived .NET applications have been forced to change.