Staying with .NET Framework is a factor that increases talent attrition and the cost because less people are willing to work with it. This is without discussing technical downsides.
You're assuming a workshop where there's just one significant app. The last several places I've worked there were dozens of apps, microservices, and APIs; some of the old and stable APIs were .NET Framework and those never needed to be touched. We built all of the new hotness using .NET Core, Blazor, and whatever the JS framework de jour was, all without needing to touch the old, stable, and working Framework code happly humming along in its VM.
> Staying with .NET Framework is a factor that increases talent attrition
100% this. If a company is not willing to move the product forwards because "there is no need to" then I would be looking else where. .NET Core will be a decade old very soon and a company not willing to move to that tech stack is just pure neglect.