The only time I really understand that argument is if someone bought a decent motherboard with low end CPU with the intent to upgrade later. Someone going from say a 7600X to a 9800X3D (or even Zen 6 X3D assuming it's still AM5) has fantastic upgrade path without replacing the motherboard. I think a lot of people associate platform longevity as some great thing because of AM4 which frankly was an anomaly born out of how far behind AMD was.
Personally speaking I don't tend to upgrade platform unless I'm starting to run into performance issues. I'd originally planned on updating with ADL, but didn't particularly like the heat output (which RPL didn't improve on). Zen on AM5 has a bad IHS which makes the chips run less efficiently than they could so I just kept delaying an upgrade. This time the deciding factor between AMD and Intel was just platform features rather than potential future CPU upgrades.
For me since decade ago I believe the MB "grade" isn't as important as long as one isn't going to put it into extreme overclocking. the chipset used is more of the decisive factor.
Back in the day when MB have North and South bridge, the MB did affect the base performance, overclock headroom and connectivity, but since we moved to multi core and esseintially moved the north bridge and memory controller into the CPU, the board mainlly affect the extra power into the CPU for extreme overclocking, and the different between different gen chipset in the same socket become less and less prominant, like the latest PCIe lane nos.
Upgrading in socket could be drastic if one goes from zen 2 or even zen 1 to say, 5800X3D, or even like in the lower end Z690 board I bought with the 12700KF, moving to 14900K did provide significant improvement in photo conversion and quite noticeable gaming performance thanks to the larger cache and much higher clock.
But normally, if one gets say the top SKU in the Tick cycle, dropping in the Tok cycle top SKU on the old board don't really make sense, while for AM4 and hopefully AM5 when the socket spans multiple generations, the improvement maybe drastic.
One more consideration is the replacement stock availability if anything goes wrong can be diminishing with a dead platform, if luck strikes replacement stock will vary, so logically the value of the dead platform will goes down.
With all these considerations, the 12900KS still at $250 doesn't look like a deal, just reflects what it is worth at current consumer chips