That 10-year rating on most CR-123 cells is to 90% or perhaps 80% charged. As long as the unit isn't stored hot (over 140F) or tested too often 20 years is a reasonable expectation for at least 50% capacity (I'm assuming no parasitic load on the battery).

20 years is probably beyond the design life of the electronics anyway, especially in the RoHS era. And by then there's likely to be a replacement for the existing SARSAT signal protocol: the existing scheme doesn't really support GPS - GPS locating is a hack that only works in some parts of the world - and I expect that eventually someone will upgrade the protocol at some point, perhaps adding some situational information etc in addition to the unit serial number.