WCF, Silverlight... They were terrible. New .net stack is much better.

WCF had some good ideas and if you followed its best practices migrating to other networking stacks should have been straightforward. Delete your complex Bindings XML configs and just write implementations of your Contract interfaces and figure out where you need to dependency inject the new implementations. If you were deep enough into WCF you might have even had mock implementations you could bootstrap from.

That concept of "define an RPC interface and let the platform fill it" was ancient before WCF and continues to this day (I've got a bunch of tabs open for Cloudflare Durable Objects right now). The legacy problem with WCF was not but long term backward compatibility with the Bindings abstractions and XML config junk and SOAP Stack RPC. Without WCF native in the box in .NET 5+ it is harder (but not impossible) to build SOAP RPC monoliths. But so few businesses actually need SOAP and can't just switch to something more RESTful.

Silverlight was maybe ahead of its time given the growing popularity of Blazor WASM.