Simply because sw bussiness has changed, there are androids, there are core linux and so on. Microsoft have to start fight to get more manufactors using windows on embedded products and then later they may get payback in another ways.
But yes if nokia is smart they choose some Qt based OS. However Nokia has not choosed smart the latest years, so why would they change now? Because Elop era ends? I doubt... If people are so stupid thinking that Elop was the only idiot at Nokia board, well then they are naive. We can only hope the new board is not to much "american sw is your best friend" but I have my doubts...