"How long is long enough to avoid breaking something the users depend on?"

10 years - a piece of software compiled with 'supported' libraries, SDKs and what have you should function for 10 years. An appliance or a service should function for 10 years and have spare parts avaliable for that long.

That is the guideline I ise as a consumer, if I can see that something won't last a decade, I won't be spending money or time on it if i can avoid it. At that point it's no longer a durable good, it's a consumable.