"au contraire
It is much more likely that a runaway program will toggle a port pin"

Possibly, but that won't disable the watchdog. It's rather less likely that you'll get stuck in a loop toggling the appropriate pin unless you've made a real pig's ear of the watchdog strobing code.

"than it will write two different bytes consequtevely to a sfr."

Unless your program has code (called normally or not) somewhere in it for disabling the watchdog.