The situation is very different here. The core IntelliJ is open source. There're plenty of open source alternatives. So if the license is changed, there're a plenty of options to migrate project to alternative IDEs.

In case of Xamarin, there's no such choice. Only Xamarin can compile C# to iOS, and if the license changes, the whole project implemented with the tool is locked in. The only option is to rewrite the whole project with alternative technology.

I think, LGPL open source core SDK + paid IDE + paid additional libraries would be a much better choice which mitigates the risk. But Xamarin for some reason decided to choose a different path.