There is the fully open source debugger from Samsung, the Red Hat derivate/extension for eclipse and others are in the works. I'm happily debugging .NET applications with JetBrains' debugger on linux. One tool by Microsoft for the ecosystem not being open source, doesn't change .NET (Core/5+) being open source. Embedding a stripped down .NET Framework in browsers as a replacement/alternative to JavaScript, even if not required, would likely lead to the development of one or more new debuggers anyways, to have an in-browser development experience similar to how it is now with JavaScript.