> In quantities I'm designing for, it doesn't matter, and the cost is dwarfed by the cost of my time writing the software

For low quantities I do that too, but once you get into 100s of 1000s or millions of units, every $0.01 more profit takes out any developer cost. We are at a few dimes per mcu and it makes a massive difference compared to the previous mcu which was just over a $1; you can hire a boatload of developers for that and not even notice it.

Offtopic curious: from your profile: C64 BASIC, C64 Assembly, Logo, Pascal, C, C++, Scheme, Perl, Common Lisp, Clojure, ? => assume you use c/c++ for these controllers? I'm c or arm asm (our current controllers have 24kb free) but always interesting if people try to actually use anything else than c/c++. Like you say; maybe Rust, but it doesn't fit too nicely in your evolution => ? imho.