Handsets are becoming more advanced and gaining functions previously associated with the smartphone category. They therefore need a more modern OS. Nokia shipped 106m handsets last quarter; if they're paying $7.5 per handset for a Symbian license, handset sales stay steady, and more and more handsets need a Symbian-level OS, then in a few years time they'll have an annual bill for $3.18bn.
Oh alright - it won't be this bad, maybe not every handset will be SymbianOS and maybe they'll have negotiated discounts from Symbian - but it's still a lot of money. And if in the meantime processors are getting faster and others are demonstrating that you can build a handset atop an OS with UNIX roots... Linux must look increasingly attractive from a financial perspective, particularly if SymbianOS really is a "piece of shit".
Not that this necessarily means you'll be running KDE or similar on your phone (much as I'm sure that'll excite some of the iPhone bleaters out there...)