Old PSD files open with all later versions of Photoshop in every example I’ve seen. Newer PSD files include features that aren’t available in old versions of Photoshop, so for obvious reasons those newer files aren’t completely compatible with old Photoshop versions.
But overall, I think you’re misunderstanding what I’m saying. In order to fundamentally change what needs to be stored in a PSD file – which is basically a serialization of the Photoshop document that lives in memory while you work with Photoshop – you would need to fundamentally re-think the very low level abstractions that Photoshop is built in, which would mean completely changing the Photoshop user interface and every level of the technical stack, basically rewrite it all from scratch.
Given the features Photoshop supports, and the way they are built, the technical details of the PSD format are actually pretty straightforward and reasonable. If you spent a few months learning how all the Photoshop features interact and getting a loose sense of how they’re implemented, and then you read the spec for the PSD format, I suspect that you would find the file format to mostly make good sense.
The reason that PSD is complex and hard to implement for third parties (especially for someone not intimately familiar with Photoshop’s UI or structure) is that Photoshop is a very complex piece of software with many moving parts, worked on by hundreds of people over 25 years, not that Adobe’s engineers are incompetent or made the format needlessly arcane just to be jerks.
