Migrated a small-ish project from ASP.NET Framework (.NET 4.8) to ASP.NET Core (i.e. .NET 8) including EF6 to EF Core last year. Even though it was pretty smooth sailing (the application was already using MVC), it was still a few weeks work.

Rewrite queries that are not yet supported on EF Core, create some new DTOs, create drop-in replacements for stuff that isn't there anymore, create plumbing to connect some old stuff to some new stuff. Figure out what's the new replacement for XYZ and use it correctly to not break anything. Replace nuget packages that are not maintained anymore or have so few downloads that they are a supply chain risk.

You need to test every endpoint, every query in your application.

And in the Microsoft World you're still pretty ok, overall, it's quite stable.

God forbid you put your money on the wrong web framework (not even talking frontend) in the Java space. There's still Tapestry/Struts/Grails/JSF/GWT/Play etc. applications serving your request out there. Saw some banking website with ".do" urls recently (indicative of Struts).

Unless we get superhuman AI that can do such migrations all on their own, those zombies might have a longer working life left than lots of developers. Many companies just can't afford to upgrade - there's so many systems built over decades that just do what they should do. Why shell out tons of money to just get the same functionality?