You can use copper fittings over in most cases BUT WHY?
A copper 5/8 Ell cost about 10-15 CENTS each does it really pay to take the time to remove the old solder alloy from inside this CHEAP fitting?
If this fitting was properly soldered YEARS ago and IF they didnt use any type of lead solder and If you you could get all the old alloy out GO right ahead and use it.
About gate Valves failing. The proable causes are as follows
1-Using VERY cheap imports
2- Not opening this valve fully and trying to make it act s a globe valve (throttling)
3- Improperly installed
4- Water Quality has a lot to be desired
There is nothing wrong with a GOOD Quality ball valve BUT most do not have the same full flow as a decent Gate type.
I hope this helps