I don't have anything against keyed ignition and have flown a ton of them, but it's mostly a modern Cessna singe engine thing isn't it? (By modern, I mean anything since about 1970).
-Lots of old Piper singles are keyless start
-The C170 that I fly is a keyless start
-The A36 Bonanza that I fly has a keyless start
-Every prop twin and turboprop that I've ever flown has been keyless start.
It's not like those airplanes are extremely hazardous or more likely to be stolen than anything else. As far as accidentally leaving the mags hot, you're supposed to be using a shutdown checklist so that doesn't happen, regardless of what you fly.
I'm a big talker since I haven't even started building and RV yet, but when I do it will likely have a couple of guarded toggles for the ignitions and a simple guarded pushbutton for the starter, based on my theory that simpler is almost always better.
CFII, MEI, A&P, Janitor