I upgraded an entire engineering computer lab's worth of CPUs once... in 1996. lol. Those Pentiums swapped out pretty easily and Cyrix P166+ processors fit nicely into the same slots as old Pentium 60s (though even then, the motherboards were a bit of a bottleneck at times). A roughly 2.5 x jump in cpu speed back then meant a lot for NT4 boxes running AutoCAD and other engineering software.
Today... I can't imagine why anyone would bother (though maybe it's my lack of imagination's fault).
CPUs aren't usually the bottleneck for performance. Moving to an SSD and a better graphics card tend to make the biggest difference (used to be a RAM upgrade would do the trick, but now most machines come with an adequate amount of RAM and Win10 has gotten much better about memory management).
These days, most people have a laptop instead of a desktop... same story -- upgrade to an SSD, upgrade the RAM if necessary... but... many can't upgrade the GPU. So, the laptop gets tossed for a new laptop. Hopefully Thunderbolt 3 / USB C will change that somewhat with external GPUs, but I'm skeptical.
Anyway... I haven't swapped out a CPU for anyone in over 20 years. Even though it's partially due to the slot changes, even if it were possible to fit the newest, latest & greatest CPU into some of my older boxes, the CPUs would be crippled by the lack of features from the motherboards. It wouldn't make any sense to do it. Sure. Put that Core i7 into my 10 year old board w/ DDR2 and PCIe 1x and a voltage regulator that won't let it ramp up to full speed or work with power stepping properly... and I'd need a new heat sink.
The real reason they don't bother keeping the same slots is b/c features change and most people keep their computers for 7+ years now anyway. I have 2 that are over 10 years old and still play Netflix and Youtube streaming vids just fine -- even some 1080p h.265 videos without a hiccup (though would play more if I had a decent video card so they'd use the vid card instead of the cpu for h.265) They're my backup and testing machines as I've moved on to gaming laptops... current one is 4 years old & I am only now considering buying a gaming PC -- to replace the laptop for gaming purposes (I want 3 monitors and streaming/video capture), but the gaming laptop will still be used for all sorts of multimedia and gaming.
But, back to the topic, what's really a reasonable time frame to keep the same slot? I'd say only so long as it's practical -- which would mean for every new cpu feature that required a change to the motherboard to enable, get a new slot.
The great part of this is that the OLD motherboards and CPUs will get a price reduction as vendors try to clear their inventory. I'll be on the lookout for great deals! Still... I'll probably get a Ryzen7 or Threadripper... b/c maybe by Christmas the prices will be more sane and will be even better price/performance compared to Intel.