Different definition of centralization. Coinbase may hold and control a lot of bitcoins, but they don't control the network and can't make fiat declarations if they want to ban/block someone from using bitcoin.
If a sufficient number of businesses integrate coinbase into their sales, it's not just a matter of the paying customer going elsewhere to process the payment: the entity selling the goods also has to move away from coinbase.
Sure, you can still send coins over the network - but given a large enough foothold that doesn't mean you'll be able to pay for the goods you want, at the merchant you want.