This is a very important point to make: data interchange formats typically much slower to manipulate and require much more RAM to do so, as they will require mapping back and forth to the internal data structures actually used at runtime. If you take a look at an older (pre-"x" formats) Office document, you will find a lot of its complexity (in addition to the aforementioned backwards compatibility and "numerous unrelated features" issue) relates to figuring out how to edit and resave enormous documents quickly.

Exactly. I clearly remember working on a 20MB PSD file in the mid 1990s (a restoration job done on a borrowed Mac of a badly damaged photo of my grandfather; the picture still sits on my mother's dressing table.) That took an absolute age to load and I know there wasn't much more RAM than that in the machine. It worked well enough though.

Old formats like PSD are better viewed as archaeological artefacts than as exemplars of some elegant ideal.