I own one of the new Macbook Pros, and the build quality of them is fantastic. It's the sturdiest computer I've ever seen. It's a huge jump from their previous models, it's even better than the 2015 models, which were already very good. I've opened it up to look at the internals, and there's so much material everywhere. No wonder it's heavier. Even the base plate is a lot stiffer than in the previous models.
Now, I must concede that I'm not too familiar with windows laptops, but I've never seen anything remotely comparable to this. If you could point me to a 1300€ windows laptop that's even close to the Macbook Pro in build quality, I'd love to hear about it.
And that's not even mentioning how fantastic the display and the speakers are, because they are unbelievable.
The only thing that's cheap about the new Macbook Pros is the camera, which unfortunately is still a piece of crap, just with some machine learning algorithms applied (they don't help).
Obviously I don't Apple's exact margins, they are probably lower than 50% but not by much however: "But there's no way they could keep their entry level prices as low if they charged a fair price for upgrades." is simply just not true, they obviously could do that if any of the conditions I mentioned changed, they just don't because as any other company Apple seeks to maximize it's profits.
Dell XPS, Thinkpad X1, Razer Blade and Surface Laptop lines have pretty decent build quality, of course they all have inferior CPUs which use more power and generate way more heat compared to the Mac (I didn't say you can get one for 1300€, just that that other manufacturers try to price their premium laptop lines at similar level as Apple but usually end up having to sell them at significant discounts)
Not that I'm trying to blame Apple, they've done a great job and it would be irrational for a company to not take the free money (I'd do the same and it's not that their competitors are better, high end PC laptops have similar margins it's just that less people want them so Dell, Lenovo etc. tend to sell them at massive ~50% discounts eventually (same with Android phones) which Apple never does, since their prefer to sell at relatively low volumes but with huge margins). However their prices for ram/storage are objectively predatory and the only reason they can get away with it is because there is no viable competition, it has nothing to do with "subsidizing" entry levels Macs. If they wanted to they could easily cut their prices by 500-1500 across the entire 14/16" mbp range and still remain more profitable than all the other laptop manufacturers.