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Price of comfort for having a platform. Or not even comfort really? Just the basic to even be able to trade in the first place. They're a business at the end of the day, so they still gotta live off of their users. But if the fees are indeed that high maybe try to find another platform to trade in? Pretty sure there's a basic metric you'd use to judge fees and personally I've never had that big of an issue with where I'm trading with right now.
And no, bonuses and discounts are just that, bonuses. I don't think trading platforms need to add anything to make it more sustainable, it already is sustainable.
As it was said about Binance and others - there are possibilities to get the fees lower on these platforms, but I do think you are right, if you use something and it's centralized - the service has its price.
However, I never calculated how much I got from the fees if I didn't spend a dime on them. Partially because I don't buy coins so often.