It's a complicated format but that i imagine is from the fact that it's highly backwards compatable. Not always with 100% success but i believe it's possible to open new PSD files in older versions of photoshop. It's rare to see compatability work both ways like that.
There are watches today that are 100x more powerful than Macs of that era. Almost anything that was good in a file format from that era would be bad today. Every novel and innovative application feature from Photoshop 1.0 would be seen today as trivial. Many of today's features would have been inconceivable.
If you think you can do better, then I'd encourage you to try. Just start shipping an app with a file format today, get a lot of users, take it through 15 major versions [2], and come back to us in 25 years. Then you can lecture those of us left alive on the right way to do file formats.
[1] http://en.wikipedia.org/wiki/Macintosh_SE [2] http://en.wikipedia.org/wiki/Adobe_Photoshop_version_history
The challenge needs to add the condition of the app stretching the hardware capabilities so hard that a whole pile of not-so-clean optimization tricks are necessary to get the performance up to the acceptable level.
The file format is also well documented and fairly complex. https://www.sqlite.org/fileformat.html
None of these does exactly what PSD does of course, so it's impossible to compare them.
A better example is perhaps TeX, which predates it by far, has been extended a lot during the years, and can still render both original documents as well as modern ones.