People are lazy
'Most' average users are lazy, they expect the phone to update itself fixing problems.
They do notice when this doesn't occur and the same annoying software bugs are never fixed.
Still if the price is correct and not carrier discounted they'll sell well.
The problem might occur with confusing the customer base.
Nokia will be pushing a fair few phones onto the market this year, running 3 seperate operating systems, along with current phones on the market currently running their OS.
It's hard enough for people to understand the difference between a HTC running Android and a WP7 one.
Try explaining the difference with a Nokia phone which could run anyone of 4 different Operating systems.
People will just get confused and walk away.