Re: Dojo loyalty or personal gain?
It sounds like you enjoyed practice up til now at your current dojo. Maybe go to your Sensei and ask him if there is anything you can do to help out.
Changes like this happen and it may take a while for the "magic" you once experienced to re-emerge.
Regarding why the other sempai left; does it really matter? I only ask because you mentioned that you have been there for a year and you said you have stayed out of the politics although you are aware of what is going on.