Wait, that exists. USB is the standard. The UPS with standardized DC-outs is a USB power bank with integrated pass-through charging.
The problem with calling USB a DC power standard is that it often doesn't work. I have a lot of trouble trying to charge my PS4 controller using random USB cables when it's attached to my PS4--I'm not even introducing an a/c adapter. I've had things like Qi chargers where I had an insufficient a/c adapter so they behaved poorly. If I'm at an airport and need a quick charge on my phone I'm definitely not going to use their USB port because if it works it'll likely charge slowly.
USB-c is worse with all of this. With a/c power it's only an issue when traveling to other countries and in many cases a physical adapter is good enough.
The possible dark pattern I see operating instead is manufacturers are opting to ship power banks charged up to the retail shelves, and likely don't mind if people keep buying more new units because they can't wait for the units they already have to charge up?
I tend to keep all my gear plugged into chargers whenever I'm stationary because I know all of them have charge protection circuits and I have a work profile closer to that of a digital nomad than an office-worker, so I'm an outlier with that common consumer behavior of only charging when one must.
[1] http://community.anker.com/t/any-powerbanks-that-allow-drain...