"Futureproof" isn't as much of a time frame as it is whether it makes sense to upgrade that bike to newer/better spec. Like I gave in my example back in post #17; that even if it's 12-15 years old, a second (or third) -hand Ultegra bike would be a much better base for upgrades than a brand new Tourney-equipped bike, even if they cost the same money.
One example that's come up a few times lately is the mid-90's CAAD-3/4 Cannondales; their design was way ahead of their time, and the build quality was some of the best in the business. Upgrade the driveline to 10/11sp and fit modern wheels, and they're still the class of anything on the road.(for way less money)

Not "futureproof" would be those guys who ask about how to fit a 105 gruppo on to their $199 Big-box road bike (Kent, B'Twin, what have you) Sure, you can bodge enough stuff together to make it more-or-less work, but after you've swapped everything but the paint job, you've still got a cheap, heavy, poorly finished low-end frame.
Also, bikes that make use of a lot of proprietary tech, even if it's new, at some point, it'll be an orphan, and you'll have to either be really creative, or have to keep scrounging through a dwindling supply of spares. (I have an old Softride, I know this well)