"I honestly don't understand all these quality sacrifices just to make things thinner. It makes the product actually feel somewhat cheap and brittle."
Here's a hypothesis:
For many years, before the iPhone, Apple's computer business was built around keeping its loyal customer base happy. That base would buy new Macs by default, but would stop doing so if the long-term experience of using them went down too far. So Apple focused on long-term quality.
Now, their computer business is built around seducing iPhone users, most of whom who are now using Windows computers, into moving to the Mac world. The vast majority of such users base their decisions on little more than what looks best in the showroom. For that showroom seduction to occur, Macs are better off being the hottest looking computers there. Whether or not their keyboards work for much more than two weeks after purchase is far less important. (14 days being the period of the Apple return policy.)
Another factor: there is little reason for annual or bi-annual replacement of fully functioning computers any more. I'm still happy with my mid-2012 MBP. In the old days, Intel chips got faster according to Moore's law, and new software took advantage of that speed, so you really had to replace hardware at a fairly rapid pace to not experience everything getting slower and slower year-by-year. But the speed of my 2012 MPB is just fine. The situation is night-and-day different than it used to be.
So, once Apple achieves that initial showroom seduction, even if the user doesn't end up being particularly happy with their machine, they probably aren't going to buy another one for 5 or so years anyway. So customer dissatisfaction isn't going to be felt for that much time.
Moreover, from what I'm seeing, customer experience in the Windows world isn't consistently better, so word-of-mouth isn't particularly likely to pull the average user away from the Apple ecosystem and the way everything works together, and years of their emails and documents being stored in the Apple cloud which they'd lose access to, or go which they'd have to through the pain of moving elsewhere.
Overall, if making ever-more money is Apple's only real goal, focusing on thinness over all else is arguably a rational choice, as long as thinness makes Macs look sexier in the showroom.