JavaScript itself is entirely backwards compatible back to 2002. If anything, I'd argue raw .NET is far worse than raw JS for backwards compatibility, because .NET has depreciated entire languages like VB6.

To compare something like React to .NET, let's look at .NET libraries and frameworks. For example: Sliverlight, WebForms, WPF, WCF SOAP, old versions of EF, or old versions of MVC. It's not been pretty for .NET, and you're in a bad place if you still have a Sliverlight app in 2022 because it only runs on IE11.

Every language and library suffers/benefits from the onward march of progress. It's disingenuous to claim JS is the only ecosystem that has significant churn.

WebForms, WPF and EF 6 have been ported to .NET Core.

WCF althought not initially supported, the industry has made enough pressure, that most of it is now available on WCF Core.

SOAP libraries still exist.

Thanks to the magic of WebAssembly, you can port that 2002 Silverlight application to OpenSilver, and use it on any modern browser.

Now lets compare it to Qt and QML? They seem to be pretty stable GUI libs