It's accurate to within a meter and works perfectly fine today, without us having to spend billions more on a replacement that could surely be spent on those pesky problems like healthcare or education.
Why wouldn't we keep using it for hundreds of years to come?
IDK because European ones already work down to the centimeter...
I've just sailed into 2-3 meter wide canal after dark here in NZ. Used my previous track from earlier today. 1 meter accuracy is not precise enough to do it safely - had to look for fairly poorly lit nav marks and rocks.
I wouldn't trust GPS down to sub-meter precision even if the spec technically supported it. I often travel a route, then turn around and retrace the exact same route, only to see the GPX data indicate fairly large variances. LEO is very large and the atmosphere is not uniform.
Or you could use lights and your eyes.. Blindly trusting anything like GPS to guide you through risky situations like that sounds like a good way to ruin your boat.
The error beyond this level is due to local ionospheric conditions rather than the protocol. Corrections can be transmitted to interested receivers, WAAS is an example of such a system.
cm level accuracy is available with RTK. It works by comparing GPS signals from a known nearby location with that of a mobile receiver, providing what amounts to especially good correction. It works fine and you can get started at home for <$100.
A new protocol that does better than what we have right now will take some major breakthroughs.
The Kessler syndrome (also called the Kessler effect,[1][2] collisional cascading or ablation cascade), proposed by the NASA scientist Donald J. Kessler in 1978, is a scenario in which the density of objects in low Earth orbit (LEO) is high enough that collisions between objects could cause a cascade where each collision generates space debris that increases the likelihood of further collisions.
Why wouldn't we keep using it for hundreds of years to come?