I've got this development project underway where the code is straight ANSI C. It reads files does a bunch of magic stuff and writes files back out. For several weeks now I've been using plain old DOS running on an 10+ year old 486DX4-100 as my development platform.
The old 1989 vintage Turbo C 2.0 compiler (ANSI compliant) and Turbo debugger are completely sufficient to do this work. The old BRIEF editor is very fast on a 486. So far the program fits well within 640K and probably will for at least several more months worth of work.
As a practical matter, there's nothing currently that would make me move it to a faster machine or to a different OS. At some point in the future when basic functionality is settled and the most egregious bugs worked out I'll be making sure the code is truly portable among a slew of different compilers and platforms, but for now trusty old DOS and a geriatric C compiler are doing the job just fine.
Software is curious product in that it doesn't wear out, rot, rust, or decay in any discernible manner. Its as good (or bad) today as it was when it shipped originally.