while 'since' is not wrong, if it is followed by 'then' I think replacing it with 'because' would sound better as at the moment the reader automatically takes 'since then' together.
If you agree, could you edit and leave a message please?
If you don't agree, could you let me know?
Because of the continuation, the speaker is obviously relating to the benefits he himself derives from having principles, not from the existence of principles in general (am I being to philosophical?)
I agree with Efylove and would suggest to use "need"...
and perhaps I would begin the second phrase with "So" or "This way", instead of "Because then" (only a personal opinion )
Thank you all!
I think all suggestions here are a matter of personal preference that don't change the meaning of the text so I will validate this translation as it is.