[ubermonkey]

I mean, I get your point, but I'm confused by the statement

"trainers don't work if you have 7+ bikes in a household with differing drivetrains, even the ones that attach at the wheel"

They absolutely DO work, even then. Wheel-off smart trainers (Wahoo KICKR Snap, e.g.) don't care what your drive train is. We can put every bike we own on ours, and that includes bikes with 9-speed, 10-speed, and 11-speed drivetrains.

If you opt for a wheel-off trainer, sure, you'd probably need to ensure chain/cassette compatibility, but then you're clearly making a tradeoff for a superior "feel" and better power calibration -- in other words, you're probably racing and have no interest in keeping your 9-speed bike on a trainer when you race on 11. ;)