Unfortunately i cant give you any objective and definitive answer on what the most common prediction was back other than saying what i personally saw and remember. What i can say is that i clearly remember a lot of discussion about how bad the PS3 was selling and there were definitelly not a clear share of people who said that the PS3 would end up selling like it did. I'm only going from what i saw and remember.
You're right about that a pricedrop and the 40GB model came out later on, but in April 2008, the PS3 was starting to sell less than 10k a week again. Also, in the first 12 months (6. November 2006 to 29. October 2007), the PS3 sold about 1.24 million units according to Media Create. In the next 12 months (5. November 2007 to 27. October 2008), the PS3 sold about 1.14 million units. So the PS3 sold less in the next 12 months despite having a pricedrop and MGS4had also been released. This didnt exactly strengthen people's belief that the PS3 would reach 10 million, so it was definitelly not a common prediction, at least from what i saw of all the comments that i read.
EDIT: By the way, i can also mention that i'm mostly thinking about the earlier predictions, in comparsion to doing early predictions for the PS4 now (or whatever system it might be). When the PS3 got its first official pricedrop like you mention, it had almost been a year since its launched. I was thinking about earlier predictions that this, maybe within the 2-3 first months or so after the release of the consoles
EDIT 2: If you want to check for yourself, here are links to all the Media Create threads from 2007:
http://www.neogaf.com/forum/showpost.php?p=24968697&postcount=4
There are thousands of posts though, so it would take a huge amount of time to read through all of them, but skimming through some of those threads might give you some indication about what people though about the PS3 long-term sales