Short answer: software upgrades on Symbian phones are technically possible, but there needs to be a business model that makes them commercially feasible.
Longer answer:
The software in a Symbian-powered phone contains many additional items, beyond what's in a given release of the Symbian platform. See
here for an explanation. Some of these additional items will have internal dependencies on the Symbian version number. If someone upgrades the Symbian platform version, the overall mix won't function properly, unless the additional items are upgraded too. Then there will need to be some testing, etc. So that's additional work for someone to do. Who's going to do that work?
Could third party developers take advantage of the open source nature of the Symbian platform to do this work? It depends. Just because the underlying platform is open source, it doesn't mean all the other items in the phone are open source too. (In fact, at the moment, many of these other items
aren't open source.)
So it comes down to a business model decision. Are phone manufacturers (such as Nokia) going to move to an operating model in which they create and distribute the updates themselves - or in which they make it easy for third parties to do this work?
Personally I see this trend as inevitable. Users will gravitate towards the manufacturers that (directly or indirectly) support software upgrades. That's part of what's implied by re-naming "mobile phones" as "multimedia computers".
Even longer answer:
See
here.
// David Wood, Symbian Foundation