The current strategy (S40/Symbian/MeeGo, with Qt being the common platform for Symbian and MeeGo) is relatively new and was expensive to follow, wasn't it? The new Symbian, making Qt what it is today, developing MeeGo, Qt-related changes to Ovi and so on...

If Elop changes it yet again, it's like burning money. No matter what he changes. (Some say MeeGo remains, but they will kill Symbian. Others say changes are for the US market only. Then there's those who claim to know it's MeeGo they'll let die...)
Besides, comparing Nokias offerings with those of its competitors, it's not the OS of any single phone out there (except the N97) that's bad. What they have a problem with is
a) marketing and communications and
b) services and integration (including things like the infamous Ovi Suite and various Ovi services)
These things wouldn't change really, would they?