It's plausible if I really wanted to, I could run a separate service/application from some non-.NET-based alternative, but the integration is likely to be a lot messier. Right now, I'm not sure .NET 5 offers a compelling reason to drop .NET Framework, which Microsoft will presumably continue to support for at least the next decade.
Microsoft still lacks a cross-platform desktop UI framework, they have no project template in Visual Studio that would let you write a service that can run as a service on both Windows and Linux, and .NET 5 is missing lots of Windows integration functionality like System.Speech as well. I'd really like to make my project both run as a service, and run cross-platform, but there doesn't appear to be a good path to do either one with .NET 5 yet, and it'd likely take substantial rewriting just to get my project to .NET 5 on Windows.