I'm not even sure we **should** be looking at this .. we already only base on LTS's (so every version is good for 5 years, or whenever the user CHOOSES to reinstall a new version), do-dist-upgrades have a horrible habit of borking peoples system when they haven't bothered to backup and just "trusted" something that because of the sheer amount of variables is inherently risky at the best of times.
If they're forced to do a full install, they're MUCH more likely to backup their important stuff first.
Personally I've never trusted do-dist-upgrades on PC's with a GUI .. they're stable "enough" on the GUI-less server editions .. but evn there they go wrong occasionally.
Think about this .. would you trust a "do-dist-upgrade" on say a system with full disk encryption .. where if it goes wtrong, they're likely to have lost everything.

And just as importantly, would you be willing to offer support in these circumstances ?
I'd rather people moan that we don't have an easy upgrade path, than moan that we lost all their stuff
