My thought it that from the side of Darwinex and the regulators they probably don't want bitcoin because of the added compliance burden. A broker can't be seen to be helping people not declare income for example.
Would suggest you want to use bitcoin to keep things from tax authorities? Having a grey area with tax isn't something the regulators would want and by extension brokers don't want to annoy regulators.
In terms of this there has to be a method of holding BTC. Currently Darwinex use Barclays I believe to hold customer deposits, they would have to implement a whole new system fro BTC.
Its pretty cynical, and probably untrue, saying the dollar will collapse but I can understand your concern. However I don't agree that we need bitcoin on this platform. The majority of brokers that offer bitcoin I wouldn't trust with any of my money! This is a serious broker for serious traders and investors not a Trading212 type casino.