My 12v battery failed on me after about 4 years. Was rather inconvenient, as I had to replan a whole day and move all appointments. So I rather have it replaced by the service than by surprise.
But I also do think smart should simply use 12 v batteries which are meant for EVs. This way they would last at least as long as the HV battery and they wouldn't have to replace them. Right now they use the same battery as they used in the petrol cars. Which simply aren't made for the EV use-case.