Even if only 50% of those migrated over to Google+, if they had found a way to cleanly incorporate RSS feeds into their interface, that's probably a non-trivial amount of users, especially if they are checking multiple times a day, and sharing news/information from their RSS out to readers.
It's not just about users, it's also about activity..
Let's say they have 12M monthly active users; a large percentage of those are probably already G+ users (let's call it 50%.) A large percentage of the remainder have deliberately decided they don't want to use G+, so you'd be doing pretty damn well to convert 10-20% of them over.
So that's only 600K-1.2M MAU in a "best case" outcome. You're just not going to be able to sell that as being worth allocating a half-dozen engineers to pursue building a new product. It's not even close.
In the use case of a type of site where cascading network effects are of paramount importance, 6M engaged users is huge.
My understanding is that the better figures are closer to twice that: http://www.gwern.net/Google%20shutdowns#fn5 (and that Reader users were very active).
By integrating both services (specially the old social features from reader) they could make Google + a much more active community, overnight.
Instead, they decided to send their users to other venues. This is just dumb.
That sounds like a lot, but at the scale G+ and FB are competing, a few million (possible) users isn't worth allocating dev resources for. They're looking for things that move tens, or hundreds of millions of users. Some opportunities are just too small to pursue.