Generally the space stuff avoids updates because it's risky, but also because it's oneshot: you can't go back and have another pass at Pluto with the updated software. You have to get it right first time. But there definitely have been updates to the Mars rovers, due to the continuing nature of their missions and to adapt to hardware damage (such as Curiosity now driving in reverse).

Insistence on warranties would up the cost of software exponentially. Did you mean "obsolete" for absolute, or absolute meaning "fixed and immutable"?

certainly they are doing upgrades and maintenance. They have probably set time frame requirements for these too, for instance, "device must be able to function with current sw for N hours/days/months while out of radio contact"

Your question was a great one though -- and worthy of pursuit as a sw engineering goal in general!