Correct. If anything, Coinbase's value should be tied to the volatility of the crypto market. Doing a quick search of various crypto volatility indexes, this appears to check out, roughly.
The trick though is volatility = both times of lows and highs. So it can survive down times are well as when times are good
Not really, the margins naturally go down, Coinbase has no monopoly on trading, but Bitcoin has network effect.
Coinbase made more money on trading fees than the whole ICE exchange, it's not sustainable.