Customers would expect bug fixes, and retro fixes for breaking changes. This would be a headache to maintain. Secondly, support would become mired in difficulties with everyone running different versions.
I understand that. However it's either you don't provide these frozen versions regardless of how much users want it to feel safe for their projects or you'll put up a big warning sign that says something like "This version is no longer supported officially. We'd strongly encourage you to update to the latest version for new features, bug fixes and official support."
There will be always people who'll post topics with later versions, not understanding what that sign says. But since you constantly get questions for native exports and 3D support, I don't think that this would be such a big problem. Also, what about the offline versions? Say, an educational institute installs offline versions on their computers which don't have internet access and therefore can't be updated. When they ask for support, isn't this the same situation as someone using a frozen version?
There has been numerous examples of companies posted in different topics who do this frozen version thing. I don't think that they constantly support 100s of versions, yet they still can provide the frozen versions for their loyal subscribers. Just an idea: Make the user subscribe for a year, than only provide the frozen version when they renew their subscription. This would eliminate the problem of using old versions (since the user would update), but still would give them something to hold on. In my eyes the frozen versions' purpose would be to ease the community and give them something that can open/edit/export the projects that they made with the previous versions. The frozen version is not something that would need to be supported later on. I think anyone who is asking for such knows that, since you can't expect even a large company to support outdated versions of their software. However, providing these (at least in my eyes) would align with the open minded philosophy of Scirra.