Which is not an issue for me as 256c is my sweet spot.

Meh that's kinda overrated. There's only so many colours you can discern in individual letters.

Sure, the issue is that it doesn't gracefully degrade when presented with 24bit color.

Tmux does, though!

I used iTerm2 for a while before realising that tmux automatically maps 24-bit colors to 256color. It works well enough for me that I switched back to Terminal.app.

Well, assuming 24bit—or anything from a terminal emulator, really—is not a very bright strategy.

Depends on your colour palette :-P.