Good luck in convincing anyone that doing something that's not rational is a good idea. The very definition of rationality is something that represents something good and necessary. I don't know why you'd try and argue for non-rationality/mysterious/spiritual reasons when using something is mathematical and scientific as computers. It's a bad combination.
Also, you don't have to use the whole standard. You know there are people who use C++ for hobby projects? That standard is dozens of times larger than the C standard.
I don't know why you would consider the choice of C89 irrational. If you want the widest range of compiler portability, you pretty much have to use C89.
I doubt any programmer practices platform, tool and library choice with 100% rationality, so it's not a black and white issue. If I had in fact been trying to convince people and hadn't brought up irrationality myself, your concern would be welcome :)
Also, you don't have to use the whole standard. You know there are people who use C++ for hobby projects? That standard is dozens of times larger than the C standard.