even geometry only matters if it allows you to alter your upper-body position to reduce your wind-resistance.
It's easier to produce power when you're seated the right distance above the pedals, which isn't possible on many mountain/cruiser bikes without buying a humongous seatpost. It's easier to produce power when your body weight is centered over your pedals, which isn't possible from an upright position. And in a road position, you're innately less wind-resistant because even upright, riding on the tops of the bars, you're leaning over more than on a mountain bike. Geometry also affects comfort, and a comfortable rider goes faster. And wind resistance of the bike itself. I can go on...
Narrower tires don't make a big difference over fat slicks at the same PSI
First, yes, they do, you've got much less friction with a smaller contact patch. Second, you can't run fat slicks at the same PSI as narrow tires. Find me a 2-inch, cheap tire and tube that'll do 125 psi, please. You can't? I'm shocked.
and the benefits of stiffness and power-production-optimized-geometry pale in comparison to the earlier benefits
Are you saying they're not as significant as the road bike strengths that you just disregarded
because of how insignificant they are, or are you contrasting them with mountain-bike strengths that you haven't mentioned yet but want to make me think you have? Either way, I'm totally not following your argument.
And drivetrain maintenance isorthogonal to bicycle type.
My specific drivetrains were what I was referring to there. If you were trying to understand what I was saying and have a conversation instead of just proving yourself right, you would notice that I specifically was comparing one of my bikes to the other and not making a general statement about all mountain bikes somehow having rusty cassettes.
Everyone, in case this wasn't perfectly clear already, I wasn't suggesting you go buy an $800 bike to commute less than two miles to work on. I nowhere suggested that. I do not suggest that. I will not suggest that.