Discussing this as a "backdoor" conflates this with the usual hidden backdoor which is a bad thing. Putting in a backdoor that is freely accessible and leaves no trace of its accession is ill advised. But I fail to see why there are no technological means to secure keys for multiple parties. you can even have crypto so multiple parties must agree so for example like my safe deposit box the bank and I both have to agree that I am me.
Now that's a different question of whether
1) I might encrypt the data on my own or use a thrird party client that uses googles services but keeps things encrypted in passage. That defeats the abililty to side door googles encryption.
2) I might off shore my data to someplace outside such laws (do I trust them is another matter).
3) the dent this might cause in googles popularity outside the US--I actually doubt this since de facto it has been the case in the past that the NSA had free range of google and no one cared deeply. But Will china also demand that google also let it have side door access as a condition of doing bussiness there? Still while a mess it's not technologically difficult.
4) an even stickier issue might be who all has to agree to unlock the data. Google+NSA. Google+China. those are doable. but Google+NSA+China is a problem. China might not want the NSA peeking at chinese national accounts without it's permission. Nor perhaps North Korean or any number of disputed places the NSA is interested in.
So there's a political mess here and some ways consumers can defeat it, but I fail to see why someone like Bruce Schneir would say there's no technical means to do this at the level of google or apple or major sites when there plainly is.