[summm]

You forgot to account for much higher resale values. After 2 years, you can resell your phone to someone who can still get 5 years out of it.

[rodarmor]

The fact that consumers don't want phones with 7 years of guaranteed updates (if they did, they would be willing to pay more for them, and smartphone makers would happily oblige) means that the impact on resale values would likely be small, or that consumers are unlikely to try to resell their phones.