Do GPS watches lose accuracy over time?

No. You have gotten more efficient at cutting the tangents.

You need to understand who GPS works. It's not accurate at all when it comes to measuring running distance.

Your data points are a zic zag line. I guess if you are faster it gets more accurate because the zig zags angel not that far out as on slower runs.

More here

https://support.strava.com/hc/en-us/articles/216919487-How-Distance-is-Calculated

Do higher priced GPS devices ($400+) have more accurate speed and distance measurements than a modern $150 device? Or do they use the exact same technology and the price difference is a result of features?

No.

End of discussion.

Higher priced devices could be more accurate if they have a better GPS chip set or record your position more frequently. But that's not a given.

If something in your device is failing, then I suppose that could lead to less accurate results. But getting less accurate is not really a thing that happens. The batteries definitely degrade, though!