I think it should be a marketing consideration or what.
Because this "buddle strategy" makes you have to buy a new mobile phone if you want to experience the new OS.

But take a look at iPhone and Android, they also make a big sucess
with their OS upgrade supported.

The iPhone and Android mobile phone has a common point :
Their hardware spec is very similar and no big change between different generation products. For example, the iPhone and iPhone 3GS
are very similar in hardware spec, only the CPU and RAM upgrade on 3GS.

This "good design at first time and small change in the next generations" strategy makes the OS upgrade possible.

And let's take a look at Nokia's products, N770, N800, N810, N900

different specs, lead to different OS.

I think it's time to stop changing the spec now !

Just settle down a standard and hardware spec,

and then develop the OS based on that hardware spec,

to make the OS upgrade possible.

Better start from Maemo 6 and the relevant device.