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).