Coinbase's observation is that the cryptocurrency ecosystem is filled with many different players, with many different goals, and by facilitating commerce between them they can profit off all of them.
Your goal may be to accumulate Bitcoin and develop the technology until it takes over as the world currency. An (honest) ICO's goal is to take Bitcoin or Ethereum, convert it into fiat, and use that money to fund development of an interesting new technology. A day-trader's goal is to get rich off short-term price movements. An arbitrager's goal is to get rich off of price differentials on different exchanges.
So your goal might manifest itself in the strategy of "Always buy when I have fiat available, never hold money on an exchange, and invest in promising ICOs that look like they actually have a product and interesting technology." You buy Bitcoin monthly on Coinbase, immediately move it off Coinbase into your own wallet, and buy promising ICO tokens on IDEX as you become aware of them. This makes the price of Bitcoin on IDEX lower than on Coinbase (since you only buy on Coinbase and sell on IDEX), so an arbitrager buys on IDEX and sells on Coinbase. Your purchase was a net buy of Bitcoin relative to fiat, which makes the price rise, so a day trader takes note of that and decides he's going to sell and park his money in USDC until the arbitrager comes along and sells on Coinbase. Eventually everything evens out, and Coinbase makes a profit off every transaction.
You didn't have to deal with USDC at all, and only with Coinbase as a regulated U.S. exchange where you can exchange $USD for Bitcoin. But the other participants in this market have no moral issue with censorship or centralization, and they're all too happy to take their profit and park it in USDC while they're looking for buying opportunities.
Your goal may be to accumulate Bitcoin and develop the technology until it takes over as the world currency. An (honest) ICO's goal is to take Bitcoin or Ethereum, convert it into fiat, and use that money to fund development of an interesting new technology. A day-trader's goal is to get rich off short-term price movements. An arbitrager's goal is to get rich off of price differentials on different exchanges.
So your goal might manifest itself in the strategy of "Always buy when I have fiat available, never hold money on an exchange, and invest in promising ICOs that look like they actually have a product and interesting technology." You buy Bitcoin monthly on Coinbase, immediately move it off Coinbase into your own wallet, and buy promising ICO tokens on IDEX as you become aware of them. This makes the price of Bitcoin on IDEX lower than on Coinbase (since you only buy on Coinbase and sell on IDEX), so an arbitrager buys on IDEX and sells on Coinbase. Your purchase was a net buy of Bitcoin relative to fiat, which makes the price rise, so a day trader takes note of that and decides he's going to sell and park his money in USDC until the arbitrager comes along and sells on Coinbase. Eventually everything evens out, and Coinbase makes a profit off every transaction.
You didn't have to deal with USDC at all, and only with Coinbase as a regulated U.S. exchange where you can exchange $USD for Bitcoin. But the other participants in this market have no moral issue with censorship or centralization, and they're all too happy to take their profit and park it in USDC while they're looking for buying opportunities.