This is an excellent point and I am worried that this may be true future of bitcoin....centralized banking.
Look at the prominence and dominance of Coinbase and emergence of Bitpay. People don't want to stray to far from their American Expresses and Visa cards and I am concern that this type of bitcoin use will be the "norm". Plus, it adds stability for merchants and helps them mitigate the current volatility of Bitcoin.
I think so many people are keeping their coins on exchanges and other online services because they've come into Bitcoin to get rich quick, they don't understand why it was created and its core values, and "decentralization" is just a buzzword for so many people. And what it is worse, when people trade security for convenience, they get neither - Coinbase have ridiculously high withdrawal fees because they use bad fee estimation algorithms, don't use SegWit and don't batch their transactions. This is just one more reason to stay away from centralized services - when they get too big, they stop caring about their customers.