In every previous MacBook Pro I've owned, after a few years, when the machine starts seeming constrained by the amount memory and storage it had, I unscrew the bottom cover, unscrew the original RAM and storage, and replace them with third party parts that are 2+ times the capacity of the originals, giving the machine a new lease on life. And when buying those parts, years down the road, the going rate for disk/ssd storage is drastically lower than the same capacity would have been at the time of original purchase. (I also did this for numerous relative's Apple laptops.)
When all the parts are soldered down, you have to pay up front, for as much RAM and storage as that machine is ever going to have. At Apple's premium prices, but also at prices based off the costs today, rather than the drastically lower costs a few years in the future. It means spending more money now, and it also means guessing how much storage you'll ever need (how much you'll regret not getting originally, a few years out in the future).
And to your point of "If the machine is/isn't user serviceable, you'd take it to an Apple store to get fixed"... try taking a modern soldered-together MBP to an Apple store and saying, "I'd like you to upgrade the RAM/SSD please" - you won't be happy with the answer - it'll involve buying an entirely new machine, even if the screen / keyboard / CPU of your current machine are fine for your current needs.
I don't think Apple is soldering these parts down, in order to force people to buy new machines (I know that's a popular theory here), I think they're doing it because it makes the machines a tiny bit more reliable (a part can't jostle out of a connector if there is no connector and the parts are soldered to the board), and because not having the connectors takes less room. Well, I don't mind having my laptop be a quarter inch thicker, if it means a removable bottom cover, and replaceable batteries, RAM, and SSDs, and room for more ports along the sides of the machine. Those were qualities of all of Apple's previous "Pro" laptops, up to a certain point, that seemed to justify the "Pro" moniker.