As with many things, time has a way of changing our perspectives. No, Office formats would not have been competitive on earlier machines if they used a lexer/parser. Office documents weren't really documents in as much that they were memory snapshots. This is why early Office products had such high productivity over other competitive formats. If they were being written by scratch today, the formats would be very different, but then they wouldn't be as feature rich as their predecessors.

One thing overlooked is that the transition to the new DOCX formats (and PPTX, XSLX, etc.) was that they were being written about the same time Windows was working on Longhorn. Longhorn featured WinFS. WinFS would open the XML document formats and save assets like embedded images as their own components in the filesystem and then would reassemble the documents as you moved them from a WinFS store back to a traditional filesystem. The XML formats were in a similar way designed to make that process fast, just as the binary images did for earlier versions of Windows. The XML document formats were designed to integrate with a filesystem which never shipped. Without WinFS, the formats weren't built for third party interoperability, but were designed to make it easy to decompose a document into block components which the Office apps otherwise handled like verbose but compressed binary formats. If WinFS had shipped, the OOXML formats might not seem as convoluted.