SARSAT PLB standards do not change rapidly. There are significant international obligation entanglements that give the system considerable inertia.

Also there is a substantial investment in the private and public sectors in beacons and many people to oppose unnecessary changes. If you're forced to replace your SARSAT PLB there be a lot of unhappy airplane and boat owners, and the Navy won't like it either. Not to mention that satellites are expensive and incompatible satellites difficult to fund.

To give perspective, the SARSAT system went live in 1982 and I believe every beacon that worked then would still work until February of this year when some older frequencies were cut off: 26+ years.

There are likely to be advancements and extensions that will be desirable. But a PLB purchased today is likely to be usable, as specified when sold, for the life of the unit.