Not necessarily. The major hurdle with bringing WP8 to current WP devices is that the drivers all have to be re-written to suit the new Kernel, which would probably take roughly as long as making a new phone from scratch. It's feasible for Nokia to have been working on that, but it would have also taken time away from bringing new phones to market. Then, even if that were done, it likely wouldn't be a simple upgrade process through Zune, it'd probably have to involve reflashing the phone with a risk of bricking.
The fact is it simply requires more manpower and resources that the OEMs would rather spend elsewhere.
That said, I think the Lumia 610 probably actually is too weak to run WP8 (while the Lumia 710, 800 and 900 actually aren't), so yes you're right, we'll probably see some more WP7.8 devices being sold in the sub-$150 price range, but I don't think it'll be quite as significant or widespread as you're suggesting.
The word 'Pentaband' means '5 Bands', from the Greek word 'pente' meaning '5'. For a phone to be pentaband it has to support 5 bands. If the phone has AWS support, it doesn't automatically mean that it is pentaband. The reason Wind and Mobilicity users like pentaband phones is because the reverse is true. We're not the only ones who like pentaband phones though, so please stop referring to phones that work on Wind and Mobilicity as pentaband. It causes unnecessary cofusion.