> The writing was on the wall a long, long time ago!
The vast majority of ASP.NET Web Apps could not have migrated to .NET Core 1.0, it was missing too many features. It was missing types like 'securestring', and had zero support for Workflow Foundation, Windows Communication Foundation, etc...
At the time, .NET Core was also advertised as "an alternative platform for Linux apps", not as a direct replacement for .NET Framework.
It was only in .NET 5 that Microsoft changes their tune and started calling Core the "replacement". At the time, something like half of complex enterprise apps might be able to migrate across, but they would have encountered a long list of breaking issues with a note saying "we might fix that in an upcoming major release". That's after a partial rewrite.
You can't go to a business that has a web app that's "not broken" and suggest migrating it to a definitely broken platform that's very much still playing "catch up".
Support is improving in .NET 7, and the upcoming .NET 8, but it's definitely not 100% and pretending that it's the "end users' fault" for not jumping onto an incomplete and buggy platform is not helpful.
I'll list some random GitHub issues for you to perouse. For large enterprise apps, many of these are showstoppers for incremental or seamless migrations. The workaround is always "rewrite everything from scratch using wildly different technologies that aren't direct replacements."
IIS app pool recycle throws 503 errors
https://github.com/dotnet/aspnetcore/issues/41340
OData core libraries now support OData v4
https://devblogs.microsoft.com/odata/announcement-odata-core-libraries-now-support-odata-v4/
(.NET Framework is v1-v3, and Core is 4.0+, a breaking change!)
Workflow Foundation didn't start getting migrated until .NET 6, and not by Microsoft!
https://github.com/UiPath/corewf
dotnet-svcutil ignores most of the settings
https://github.com/dotnet/wcf/issues/4887
dotnet-svcutil silently failing to deserialize responses
https://github.com/dotnet/wcf/issues/4163
Reuse of Types not working with WCF dotnet-svcutil tool for .NET Core
https://github.com/dotnet/wcf/issues/4277
Visual Studio 16.8 breaks SvcUtil build targets
https://github.com/dotnet/wcf/issues/4431