To be fair about this jump from Maemo 5 to MeeGo/MeeGo-Harmattan, it is a big step that end users and developers might well consider a brand new OS. Qt is being really useful bridging the gap with its cross-compatible API.

Are there precedents in the industry of a device vendor supporting officially such radical upgrade?

The MeeGo project will have an evolutionary open source platform with regular and public release plans. There will be different vendors shipping MeeGo devices, and users will be in a better position when choosing the purchase that will give them the vendor software and support they are looking for. Nokia is a core promoter of this plan.