For most of the history of computers, raw CPU speeds were doubling roughly every 1-2 years.  Then about 10 years ago, chip manufacturers hit the wall in terms of increasing raw speed and had to move to multicore architectures instead.  Dramatic increases in raw CPU speed are usually easy to take advantage -- just run stuff on the new processor.  Multicore is a bit harder.  For something like low latency audio processing, as an example, you can split out processing of different tracks between the cores but you wouldn't be able to do much processing on a bus until all the separate tracks feeding that bus have been processed.  Google for Moore's Law if you aren't familiar with this and want to know more.  
 
Long story short, that's why you can get away with older PCs nowadays.  If you have a decent machine from 5 or so years ago and aren't using a ton of plugins on a ton of unfrozen takes then the incremental increases in raw CPU speeds aren't going to be very noticeable and you are already mostly taking advantage of what multiple cores will give you.  
 
Looking forward, it's harder to predict.  Intel keeps promising a major breakthrough but then keeps pushing those dates back.  
 

I have been building computers since the Pentium 3 days with Win98.  I think we have seen the bottlenecks that induce a desire to upgrade have shifted with each generation of hardware.
 
Back in those days it was a struggle to get a multimedia PC to run smoothly, as there were potential problems everywhere.  AGP and ISA slots anyone?  IRQ assignments?  LOL!
 
Then I built several single core 3.0 Ghz Pentium 4 32-bit systems that ran for 10 years, or until I retired a couple of them. Started with a good case and power supply, and used Intel branded desktop boards.  Very dependable!  One is still running at my sisters house as a print server that runs Win 7 24/7.  The death of the AGP port hasn't helped much there.  Those AGP GPU's are getting hard to find.  But other than one power supply and several GPU's no issues with these builds.  Except for the rest of the world moving on to dual core and higher CPU's.  Plus the need for more that 4GB RAM.
 
Which is why I finally threw in the towel on my main desktop and upgraded to a 64-bit Intel Core based setup.  I had noticed for a couple of years recently that greedy software developers were maxing out the CPU thread on a single core, for even simple tasks like web surfing, or running real-time AV protection.  And the number of processes running was increasing all the time.  I have an old XP install that runs empty with around 25 processes.  Win 10 runs about 70+!!!
 
But I think that we have finally gotten to a good place with hardware architecture that has eliminated many of the previous bottlenecks.  The move to multicore CPU's, plus eliminating the Front Side Bus [ https://en.wikipedia.org/wiki/Front-side_bus ], and AGP slots, legacy PCI slots, USB1.1, and so forth were big improvements!
 
The holdups are no longer typically in the CPU, chipset, memory controller, motherboard resources [PCIe slots, USB 2/3 ports, SATA ports, etc.].  No matter what I do now, I do not seem to ever redline any of these components as far as designed performance.  So as far as I can see everything in my current build exceeds my needs as far as performance.  Even an integrated HD GPU works well now, even for dual monitors at 1920.
 
Maybe the only exception here is one SSD drive I have that could be faster on a 6Gb/s SATA port that my current 3Gb/s port.  But it's fast enough!!!
 
I suppose the only upgrade I am thinking about now is additional HDD storage space.  You can never have enough of that!!! 
 
So that was a good question about how long this will last until we need the next generation of hardware.  I think we have seen some great leaps in hardware these past few years, so unless there is a game changer in software, maybe it will be a while before software will catch up to Moore's Law again. 

abacab
I have been building computers since the Pentium 3 days with Win98.  I think we have seen the bottlenecks that induce a desire to upgrade have shifted with each generation of hardware.
 
Back in those days it was a struggle to get a multimedia PC to run smoothly, as there were potential problems everywhere.  AGP and ISA slots anyone?  IRQ assignments?  LOL!
 
 

I'll see your ISA slots, and raise you one VESA Local Bus slot. My first ever build involved an ATI Mach64 VESA card, and (I think..) a Cyrix CPU. It might have had a Quantum Bigfoot hard drive, too. I used it to play Doom. And its probably in my garage still.

Amicus717
 
I'll see your ISA slots, and raise you one VESA Local Bus slot. My first ever build involved an ATI Mach64 VESA card, and (I think..) a Cyrix CPU. It might have had a Quantum Bigfoot hard drive, too. I used it to play Doom. And its probably in my garage still.

I found Doom 3 on sale for cheap at Best Buy a few years ago.  It runs well on XP and Win 7! 

I think the efficiency of how data is manipulated is getting better. It still gets the job done but I need more modern parts. DDR4, Intel Core i7, suitable motherboard, SSD boot option and another couple of SSD for programs and storage.
 
My current SSD will get formatted and reused for another purpose.
 
I've also been deliberating my cooling options. Do I go for a No-Fan solution or another 12cm heatsink option. Watercooling seems a bit risky and expensive for me.
 

I think I would rather stick to this type of solution.  Heat will shorten your electronics life.  If no fans to move air, your stuff will run hotter.
 
http://www.quietpcusa.com/Quiet-Computer-Fans.aspx
 
http://www.quietpcusa.com/CPU-Cooler-Intel-LGA-1155.aspx

You need a well vented case firstly. My current has pretty good airflow.  
 
I have front fans currently not in use, an exhaust fan in the case. 
 
Currently averaging 33.1 Centigrade (CPU), not sure about ambient. But anyway less heat produced by hardware should in theory become a more stable system. 
 
This is a point of interest for me at the moment.
 
http://www.nofancomputer.com/eng/products/CR-80EH.php
 
The plan is an exhaust fan behind the heatsink as normal and possibly enabling the front fan to increase the airflow throughout the case. However I expect using SSD storage in place of HDD will reduce the heat and power consumption.
 
 

Heat only affects stability if it's out of safe range.
 
ie: If you're CPU isn't anywhere near TJMax (when under load)... say hitting 65-70 degrees C under 100% load across all cores... then running 15 degrees cooler won't affect stability.
It certainly may affect noise...  

I need to do some research. Just looking for a quieter system not 0dB no noise at all.

Why is Heat Bad for Electronic Devices?
http://www.dowcorning.com...%20-%20Wet%20Dispensed