I wouldn't downgrade. I think its not a wise decision even if you think it is now. Yes, you might get some money but eventually, you will probably regret it and repurchase it so more money lost. The new Macbook Pro is outrageously expensive so maybe buy older model for less? Or get a part time job to make some extra money? Not sure what your situation really is but I wouldn't go from amazing cam as D750 is (and FX) to DX and repurchase some stuff again. Its just not very economical and in the long run probably not very effective.
Its up to you but I look at things as investment so now that you have your gear I would keep it and try to find another way to make some extra cash. Also, what are you planning to do on the new MBP? I'm sure older model will suffice and those are much cheaper to get.
Think about it, try to think long term rather than short term. You might be glad you did.