Each person's needs will be different and will determine if and when they need to upgrade. A simple office pc or home pc used to print coupons off, surf youtube or other websites will remain useful a lot longer than someone who wants a cutting edge gaming machine.
Personally I don't find frequent upgrades to be very economical. A cpu typically lasts me a solid 4yrs, sometimes 5. By then enough time has passed for an upgrade to be worthwhile. Storage upgrades are done on an as needed basis. Same with ram if increasing memory size due to need, 4 to 8gb or 8gb to 16gb, that type of thing. Graphics card, it depends on the games I'm trying to play and how well my current card is performing.
I usually get 2-3yrs out of a gpu using mid range cards but I'm not a hardcore gamer playing games the day they release either. It's what works for me but may not be what I'd suggest for someone else. My own personal upgrade path, I prefer to wait until there's at least a 50% or higher performance increase to be gained from upgrading.
I would say there's a difference between blowing a huge budget for 'future proofing' (which rarely exists) and going too cheap. Somewhere in the middle is a sweeter spot. Whether amd or intel the same usually applies. Considering starting with an fx 4xxx then upgrading to a 6xxx, then an fx 8xxx and potentially needing to upgrade the motherboard to one with substantial enough vrm for the higher end 8xxx if the user went cheap the first time around. It's a lot of wasted money. Even selling off older low end parts won't make up very much of the loss. Saving up and spending an extra $50 tends to pay out better in the long run.
Intel, same thing. It doesn't make much sense to start with a pentium g, then move to an i3, then an i5 and so on. Getting a lower end cpu to get started if there's going to be the budget for an upgrade fairly soon makes sense, such as a pentium g as a temp placeholder for an i5 or i7. So long as the person plans ahead and gets an appropriate motherboard. Likewise if it was an amd build and someone were planning on an fx 8350, get a decent motherboard for the 8350 up front and use an fx 4xxx on it for the time being rather than buying a mobo twice.
Unfortunately it's not quite like it was with parts, you used to be able to wait a year or so and buy new-ish last gen tech which was still very capable for a much lower price. I'm not seeing that much anymore. An old processor I used was a p4 northwood (3.0c). New when it came out sold for around $420 in june 2003. By july 2004 someone mentioned a price drop to $204. I picked up mine off ebay as a new oem still in the package around late 2004 or early 2005 for around $115. In roughly 2yrs the price had fallen a lot. Try and buy a 2-3yr old cpu for even half the price it was when it was new, much less 1/4 of the price.
Same thing with my gpu, an hd 7850. New it was retailing around $250. A year or so after it was released the price had dropped to $189. Almost a 25% price drop. Waiting for a sale I got it for even less but that's without factoring in any sales. In november 2013 the r9 290 was $399. Now they're anywhere from $300-380 and it's been 2yrs. Prices have fallen some but again not nearly as quick. The advantage of waiting awhile and buying last year's tech for a significant savings while still getting a lot of use out of it has changed some.
Those are just my opinions and things I've noticed in pricing trends. Anymore my buying habits have changed and I'd rather buy something new in the mid-upper range depending on my needs/budget. Before I used to wait and hold off on cutting edge because there was a clear advantage. Now that that advantage has dwindled I'd rather spend a few extra dollars and get something maybe not brand new but only 4-6mo old and then have a capable system for 2-3, maybe 4yrs.
In gaming the gpu seems to need an upgrade before other parts and depending on the power requirements may involve a psu upgrade as well if it wasn't planned for ahead of time. When I replace a psu I tend to buy more than I need so if I make an upgrade in the next year or two I'm not stuck buying another power supply because I'm 50-100w short.