iMacs and computers in general are only future proof for the duration they run your stuff.
If you play the most demanding games all the time, or run software that stresses the hardware, then any computer will not satisfy your needs too long, as the apps you use get updated or you get more demanding games.
To the average user, I'd say that it can easily last a long time. FWIW, I have a G4 cube that is running office and photoshop and while I don't have a ton of ram, or multitasking, its performance is great.
To my futureproof is a myth because people upgrade when they want to, or when they see a machine that has components that they want. Vary rarely do people upgrade because they have too.