I could go with a double MTB drivetrain, but I quite like 9-speed for its durability. Plus, I sort of feel like experimenting with a 1x9 and the new Sunrace cassette will allow me to do it. One cassette is about half the price that a 40t expander cog was a year or two ago.

There's also the issue of my knees, which have some recently diagnosed cartilage damage that may be the beginning of osteoarthritis. My doctor has recommended cycling on flat ground with high cadences and spinning up hills without standing, so lower gears will help with that. I need to take as much pressure off my knees as possible. It is a bit of a bummer, but we all get older and have to adapt to changing circumstances.

9 speed stuff is also agreeably cheap and I have become something of a retrogrouch for recently "superceded" bike components like 9 speed drivetrains and QR dropouts for road and light doubletrack riding.

There's no huge urgency to the change, but I am very seriously considering it and starting to work out how I can do it and maybe build a small stash of parts.