Kurt is still a slam dunk to 23XI.
As to spotters - I would say that most of the time a driver will get the spotter he/she wants. But there are certain occasions where the team will overrule him/her for whatever reason, and more times than not the driver will be ok with it (or at least try the spotter the team prefers). There are very few "fall on your sword" spotter/driver loyalties to the point that there's a fight over it. The combo either works for both parties or it doesn't.
As to why there was a switch at Penske (or anywhere) - honestly, it could be for any reason, and might not even be about racing or results. A lot of times a relationship just falls apart personally for whatever reason. The Beatles are arguably the most successful band of all-time and for some reason decided to break up. It doesn't matter how good the music is if they don't enjoy the process.