That is really disappointing to hear. I am just shy of diamond too and in the UK. I also have had very limited or no success in recent years in appealing to seabourn via my respected ta in the face of large price drops (but not as large as this). It led, in 2018, to me cancelling a cruise and losing my deposit (clearly you are past this stage) and losing included flights, to save over £2000 each as they would offer nothing. I rebooked the same cabin.