Overall, future-proofing is probably not a worthwhile endeavour. The cost of buying cutting edge components is always extremely high compared to purchasing something which is merely adequate. But if the arguments above did not seem to be absolute evidence against it, there are reasons for that.
There is no refuting the simplicity inherent in future-proofing. Building a future-proof computer that will last the next five years means not having to deal with upgrades for five years. In addition, there are certain components which can last for an extremely long time. The case is a great example. A high-quality PC case might seem expensive, but such a case might be usable for ten years. Ugrading is not always an easy process, and negating the hassle can be worthwhile. People building their own PCs, or with access to these services, can mix and match future-proof and easy to upgrade parts, putting money into long term (the case) and difficult to upgrade (the motherboard) components, while saving on cards, chips, memory, drives and parts that are easier to pop in and out.
Speaking from a sensible, budget oriented perspective, future-proofing is unwise. But it is not so unwise that doing so would be foolish, and a compromised approach that future-proofs some parts of a build is attractive. Anyone who does not want to have the hassel of upgrading or simply prefers to buy the fastest components possible, either for top performance or bragging rights, wll likely find the cost of a future-proofed PC worthwhile.