Sure, they realized interoperability would be important. But by 1995 customers were already whining about how often the file format changed. Nobody would buy Office (n+1) if it would start writing documents in a format that Office (n) didn't understand, because interoperability means precisely that you can't get everyone to upgrade at the same time. So they did a remarkably good job of holding the file formats stable and backwards and forwards compatible for about ten years. Look in the Excel spec and you'll see how much work when into allowing old verisons of Excel to preserve and roundtrip future features they don't understand.
Ok, then take a look at how HTML does forward compatibility, for example. Extremely simple, elegant. Not that i defend lexable-parsable formats - I got the point that speed was important on stone-age computers. No. It's about architecture.
Here is what I think: nobody at Microsoft ever cared about making anything simpler than what they got after the first approximation, because complexity looks impressive and hence sells better. On the other hand, making things simpler requires more intellectual efforts and usually doesn't sell well, especially in the consumer product business.
