If the incentive is for consumers to buy more devices the incentive change.

I think it also has to do with the shift in computing population. It was easy to convince tech people to buy a new OS based on a feature list. When computers became more widely used, it became harder and harder. E.g. when OS X still had paid upgrades, it was very hard to convince non-tech family to buy the update. Buying a new device is easier, because the features are immediately visible to people and carrying a newer devices is also a form of social signaling.

At the same time, the internet became far more hostile and running an OS that has all the security updates is important. So, it's easier to get people to update when the updates are free.