Don't get me wrong, I'm not against upgrading existing systems, nor am I against factoring in upgradability when building them. I'm against blindly following any doctrine, especially when it contravenes one's overarching goals, or ignores relevant context.
Here's an example of an upgrade path for one of the systems I was using when I was playing Elite: Dangerous 40 hours a week (a system I still use, but which is about to be partially cannibalized/downgraded in favor of a new build):
CPU -- 3900X -> 5800X -> 5800X3D
Memory -- Hynix CJR -> Samsung B-die
GPU -- RX 6800 XT -> RTX 4090
With Odyssey performance being so profoundly memory subsystem sensitive, going from an R9 3900X to a 5800X, then upgrading the memory, then swapping the CPU again for a 5800X3D produced major performance improvements at each step. The final build, with non-GPU limited settings, gets more the twice the frame rate in large settlement surface conflict zones, compared to the original build. The RTX 4090 was added a bit later, after the path Odyssey was taking had significantly dulled my enthusiasm for the game and I found myself with time to play some other titles, but was also very useful in Odyssey for mitigating those jaggies I find so annoying.
GPU power and cooling constraints were an issue with a (barely) SFF system and the RTX 4090 upgrade, but the cost of adapting to that (new case, PSU, and cooling system) was a small fraction of the cost of the GPU itself. I just recycled the initial case and PSU for the HTPC build; bought an adequate PSU that was on sale (a Seasonic Focus Platinum 850); paired it with the cheapest, most compact, and least offensive case compromise (ASUS AP201) that could hold the RTX 4090 and it's cooling; then build a budget custom loop sufficient for the GPU (which can be way less expensive than most people seem to assume).
Other critical context here was that there were no meaningful platform upgrades possible at the time. LGA-1700 would barely have improved performance and is not particularly SFF friendly (I can do an RTX 4090 or I can do an i7/i9...both would start to overwhelm cooling and/or trip breakers). AM5 hadn't launched yet, and wouldn't be decisively better until Raphael-X. AM4 was also an unusually long lived platform. Plus I had a use for the parts I was replacing (that 5800X and RX 6800 XT are in my HTPC).
These upgrades were relatively easy choices that weren't barred or rendered impractical by other constraints. Such is not always the case. Indeed, such is often not the case.
Some constraints close off most practical upgrade avenues, and that was the context for my laptop recommendation in prior posts. What low-profile and low-power GPU is one going to upgrade to that is going to beat what one could have had in a mid-range gaming laptop from the start? Why bother building a custom UPS to fit in a mini-PC when that's what the battery in a laptop already is? What Intel platform is going to accept a meaningful CPU upgrade, if one didn't waste money handicaping one's self by low-balling the original part? How are you going to match the power efficiency of a single board computer, made from parts binned for efficiency, with desktop components, that, at the very least, have multiple PCBs and power conversion systems setting a much higher energy floor? These are semi-rehtorical questions. The only way to beat a sensibly chosen laptop, over any arbitrary span of time, under Ameeleean2's implicit constraints, would be to spend way more money, or invest a huge amount of extra effort adapting/fabricating custom parts to fill a niche that a laptop already fills.
A laptop isn't very upgradable, but upgradeability is a means to an end, not an end in and of itself. If the journey is the goal, that's great...no one can put a price on the experience except the one experiencing it. However, if the goal is to have a computer that does what the afforementioned custom build will do, the laptop route would get there, and stay there longer, with better performance, lower power, and probably less money.