As with desktops you have options here; MXM GPU will sell at roughly the same price as it's initial value (check what used MXM cards are selling for now on the bay) and but a 3080. Or sell one laptop and buy another. Neither present any particularly great challenges.
The reason MXM gpus retain their value is because they are comparatively rare and imho a total rip off , i mean a second hand MXM gtx 1060 £450 ?. and average of £350 for a 980m ??????
I run a EVGA SC17 with a 1070 and a 4k gsync screen, its great for gaming on the go but i wouldn't replace my desktop with it, its hot , noisy , and pretty fragile.
And the biggest problem with laptops ? parts, nothing is standard, what happens when one of the weird shaped rubber feet fall off ? or a hinge breaks and damages the bezzel or the lid ? and even if you can source the parts its probably not worth it due to the price.
Pretty much all laptops even if your mental enough to stump up for a incremental gpu upgrade are built as disposable and not friendly to upgrade or repair. One of the most compelling reasons for a desktop is that its easily upgraded and repaired and for the most part is free of proprietary components.
So the tldr version of this thread so far is that laptops win the portable category (not including clevo based slab sized space heater and other equally retarded niche laptops )
Desktop wins , thermal performance, reliability , ease of repair, better comparable performance, and price.