Your charitable estimation of Apple's competency and baseline rationality is a reasonable one and absolutely could be correct, and I agree it would follow that your priors are incorrect and/or it's actually not simple at all.

But I find jdietrich's argument totally plausible. They could have tons of negative user feedback that they ignored. "They" probably being a handful of designers (so the total number of talented people at Apple is basically irrelevant). Apple's always had a certain arrogance. They (believe they) know what customers want better than their customers do: in this case a thinner and thinner laptop. Sometimes they're right. Sometimes they lose their way and I end up with a really expensive MacBook Pro that I hate typing on.

I've certainly seen a few cases inside Google where a small team ignored dogfood feedback from other Googlers ("you aren't the user", basically), then were shocked when they got the same feedback from real users. It's absolutely possible for a small number of people inside a giant organization to make decisions that later bite them. I don't have any particular reason to believe Apple's immune to this.