If they go defunct or if the C# debugger somehow becomes unavailable, that’s a strong incentive for someone else to build one, given C#’s market reach. Perhaps that person would even be kind enough to open-source it.

Either way, because C#/.NET and Roslyn are open-source, the likelihood of .NET shops being left high-and-dry without a debugger is very slim, since it wouldn’t be all that hard for an experienced developer to build another one.