I highly doubt it's even close to 90%. If it were, I think you're right that it's much less likely it would have been shut down. There are many very technical people out there that just don't find RSS fits with their workflow.
Personally, I have a list of blogs I like to read in a couple bookmarks folders, and when I feel like reading them, I'll go to the bookmarks and see what's up. I don't really care if I miss posts, so I have no real need for a system that will catalog them all for me. I have tried a couple times to get into it, but just found it wasn't for me. (As a means of consuming media. I do use RSS for automating various processes, as well as the occasional RSS-to-email when I really need notifications for something (like rare classifieds posts I'm searching for).) Anyway, my point is, I'm pretty technical - I run a web based company and embrace plenty of modern technologies - and I never got into RSS. I'm fairly certain the majority of Google employees don't make regular use of an RSS reader either. (Although I imagine a significant minority do as well.)
You think low-level employees can stop a change like that? I talked to a Googler who used Reader; he's very upset about it, obviously, but said they internally communicated the reasons they were shutting down the service and they made sense.
What % of Google employees do you think are also Reader users?
edit: I assume it is something like 90%. Which is why I'm pretty impressed that this change went through.