Since you only have to do transactions to put a balance on coinbase and then can spend it over multiple transactions, does that help scalability?
I firmly believe in keeping your own private keys but I think it's an acceptable cost to give up ownership of a small fraction of your bitcoins and trust coinbase so you can do a few everyday transactions.
I don't know where exactly to draw the line but it would be 0.5-10%. Obviously a lot less if you have a large amount of bitcoin.
Are those services good for the ecosystem in total? How does it all work? Who gets the processing fees from the merchants running the Point of Sale?
Essentially, I think that you're suggesting the fact that people use third party hosted wallets instead of actually storing your own private keys on a desktop wallet like Electrum.
Even though yes, you might save a few mBTC on transaction fees, it's really not going to be that different considering that to save on transaction fees you actually need others to have coinbase accounts as well to conduct the off chain transfers. Besides, bitcoin transaction fees are extremely low right now that I don't even think it makes that much of a difference.
But the real question you need to ask is why would anyone give over control which is one of the major benefits of bitcoin, of decentralisation? Bitcoin debit cards doesn't help scalability on-chain scability at all.