Why would Nokia care?
They're big enough that any internal standard e.g. S60 becomes a de facto standard just through sheer volume.
The other thing is that if you lock something down to a standard - particularly a complete platform - it becomes stale and dies. How exactly do you introduce something new to the market if the 'standard' can't cope with it?
I would guess the reality will be the current situation - the hardware is more or less standardised, as is the software platform. Each device is slightly different, but that's what you expect with evolving designs.
The PC platform isn't enormously different - there is no real 'standard' platform, everyone builds things differently, new bits are added all the time, compatibility is something of an afterthought and those who sell the most create the standards though sales volume.
So I really can't see a standard phone platform happening beyond the current manufacturer internal standardisation.