Does anyone know what happens to Apps attached to a model that gets archived?
My guess is that the Apps are not longer available, so best practice would be to re-point the pages to a different model before archiving the original model.
Yes, you are right. If the model is archived then all the pages linked to that model can't be seen in pages section of an app and if the entire app is linked to one model only then the whole app will be vanished from the app tile screen.
However you can easily change the source model of an page by going to Page Settings and enable multiple model options and change the source model of each page but there is a catch - you have to change the source model of a page before you archive the model. But I still believe this may not work in some cases where there are space issues. For Example if the Model is 100GB big you can not have another model of same size within the same workspace (Assuming you are not using Hypermodels) which mean you can not change the source model of a page within the App) .
Post your idea on idea exchange forum, will upvote for it.