I know the architecture too good, more than I need as I was leaving MS recently. This tweet is not likely to happen. Talking about desktop apps, this doesn't make any sense. Office apps have millions of lines in code and many years of lost knowledge between the classes. Just too many screens, checkboxes and features. A complete bloatware. Buy MS doesn't take any feature off the app- if you have 1B users, most likely…
Not in terms of code bloat/complexity (these exist in both, as you've just described), but in terms of engine size, drive-ability, edge-case handling, etc.
It's probably a hard question to answer; it's (for some reason) hard for me to articulate exactly what I mean too.
And regarding Excel-literally-in-the-cloud... is that why Microsoft needed to make Azure? :P