The point where Microsoft Word overtook WordPerfect was when Microsoft released Windows 95, which somehow broke WordPerfect, and refused to give Corel any access to it in advance of the release. By the time Corel had figured out how to make WordPerfect run 6 months later, it was too late, Microsoft Word was more widely used.
Incidentally the original formats were literally a dump of in memory data structures. Which means that compatibility became a huge headache every time Microsoft tried to change its internal data structures. That's WHY they migrated to the docx format.
that's the container format. But the actual document data is still a big mess. Although that's probably unavoidable if you keep a file format for a long time. PDF and Photoshop files are also a mess.
Office was probably feature complete in its second or third iteration. The only substantive changes 99% of users notice are in interface overhauls.
I feel like that's what Google does with chromebooks and their editor suite
Word is awful at professional typesetting, but it is really easy to use casually for making resumes/letters/etc. Excel is excellent for making lists of things and basic accounting/graphing.
Aside; the original Microsoft Office formats (e.g. doc) were formulated in C header files (i.e. a serialized array of C data types). They were garbage. Current Microsoft Office formats (ending in "x" e.g. docx) are night-and-day better (they're standard zip files with boring XML files within). People should go unzip a docx file and check it out, very not-intiminating.