I would argue that for an infrequent upgrader, chipset longevity should be even less of a factor as by say 2023 I suspect that any budget motherboard is going to likely need replacing anyway. I get where you are coming from i.e. it would improve the chances of finding a decent second hand cpu (say a high end 2020 model) but who knows what other stuff you might miss out on in terms of new connectivity, faster storage etc that could come available in five years time. I've been upgrading PCs for over 20 years (typically every 1.5-2yrs until I got Ivybridge) and not once have I ever bought a cpu without also buying a mobo.
I am an infrequent platform upgrader,and during the period after Haswell launched my socket 1155 motherboard went kaput just outside of warranty.
I have a mini-ITX IB based system,and the few remaining reasonable boards had gone to silly prices since they were not made anymore,and I ended up having very little choice at £100 to £110,so ended up having to get a B75 one which had worse specs than the one I had before and the price was not even that great(probably down to production ending). If it had been later,I would have had to get a secondhand motherboard with no warranty or get a POS H61 one which would lack even PCI-E 3.0,and even secondhand the prices won't be always good.
Also getting a new CPU and motherboard would have been pointless,since I was far more GPU limited than CPU limited(I had a GTX660 at the time),which I suspect many gamers are as they are on 60/60TI series Nvidia card and slower and the AMD equivalents.
So for me,its not only about CPUs,but the fact that since AM4 will be supported for years,even if the motherboard goes out of warranty within a few years,and goes kaput,you can get a replacement much easier.
For a lot of enthusiasts here,they want the latest and greatest,and plenty will ditch hardware within 3 years,to get new shiny especially if they are always buying higher end graphics cards.
Plus,you need to consider many gamers are running slower cards,only with a GTX1080 do I feel my IB Core i7 is starting to be an issue.
But that is the advantage of having a longer supported platform,if you need a faster CPU you can upgrade with less hassle.
I mean my mate when he heard of CFL,fancied an upgrade from his Core i7 6700 to a Core i7 8700 due to the extra threads for stuff he does.
The moment he heard that Intel made sure it wouldn't happen,he gave up and in his previous rig he did upgrade the CPU down the line. I have a strong feeling he might get Ryzen 3 at this rate - if CFL worked in his board he probably would have not bothered even looking at a new CPU for the next few years.
Even for me,the game I have the most performance issues will have the biggest upgrade with an Intel CFL CPU,so I am biased towards an Intel CPU currently especially with an over 20% performance advantage for Intel,but I don't want to invest in a stop-gap rebranded Z270 chipset.
I would rather wait longer term for something like a Z390,or the second tranch of updated boards which have definite compatability with Intel 10NM CPUs in 2019/2020.
I really hope Intel starts going back to keeping compatability for longer.
Regarding upgrading CPUs,like a fair number of my gaming mates we started with slower CPUs and upgraded down the line. I started with a Core i3 2100 since there was no real need for me to get a faster CPU at the time and I already had scouted out my upgrade path in the Xeon E3 line once they appeared in the UK(which I was taking about here long before many noticed it) when I had a fast enough graphics card. It actually cost me less to do this than buy a Core i7 2600 at the time,and with an HD5850 it wasn't like most games I played at the time really needed a 4C/4T CPU,as again I was more GPU limited.
Heck,if it were not for my overclocked Q6600 975X Shuttle SFF rig having some issues after nearly 5 years,I might have not upgraded at the time. Plus with that rig,I had E4000 series CPUs first followed by a Q6600. The Q6600 cost silly money at launch and the 975X wasn't even the best chipset for a Q6660 so it was cheaper to upgrade to one later.That is how long I stay on platforms.
In fact amongst gaming mates,I have seen quite a few upgrade CPUs on the same board if they can,so its not uncommon TBF. Probably less on an enthusiast forum,but the word enthusiast gives it away. People are more likely to throw money at their upgrade in the first place.
PS:
Sorry for the long reply!
