Business logic ports over really well - we have been able to effectively copy and paste code and the corresponding tests between a legacy .NET Framework WinForms app and .NET Core 2.0 and later service (WebAPI or gRPC) and it mostly just works. The trouble comes when the code integrates heavily with a UI stack - that’s a lot more work to port over. It was pretty hard before .NET core 2 though. But .NET 4.8 will be supported as long as Windows 10 is - probably at least another 10 years or more.