Compatibility is a lot about testing each and every chance against all relevant bits of the code.
As I already said via conversation:
You could display a warning before an upgrade, that it's on the administrators own risk and you'll get support only in that one case when you're using all packages with the newest version (in that case still meh). So it's possible to upgrade the WSC and use it's new features like the contact form or other cool things without spending much money in something you might not actually need or you just don't have that money because you're a child or idk.
I think that would be a good/better solution.
Mixing up versions build for different versions of the Core quickly lead to highly inconsistent feature-sets and a fragmented user experience for your users.
Of course it does. But everyone could do some optical corrections if it looks strange - or buy the new version.
Especially given the unified appearance it won't make sense that the same feature in app A behaves totally different than the feature in app B, (this also true with only a single app, because the Core also provides various features directly).
Do you have an example for that? Since most things end up within Core-features after passing xyz other methods.
For example, the sitemap consists of the core API that is provided by the Core and the individual implementation in the app to have it work to it. A sitemap with Core 3.1, but Forum 5.0 would lead to a generated sitemap that includes all the Core's pages (e. g. user profiles), but without any posts and threads, because this was implement with Forum 5.1. Worst of all, this can be easily missed and users start viewing this as an error in the software ("Why is the forum missing in the sitemap?"), but in reality this is just the result of mixed versions.
Hmm, yes. But it's kinds bad example (as you're already saying), because you can implement that by using a plugin and then it would be a complete sitemap.
For the question of "why blabla?": You can tell them it's a new feature and you'd need to implement it on your own or buy an upgrade. No problem or bad image for me.