The alternative would have been two USB C ports on the laptop, then ensuring a power supply is only ever plugged into one, not both. It's doable for sure, but not a path Apple would likely take.
... to which consideration I ask: why not?
I probably did not articulate this enough before: either I don't see some engineering constraint to do that, or I don't get the design choice. Were they afraid that people would plug in the power supply on the wrong USB-C port and wait forever for their Mac Book to be charged? I can see many ways to address that, visually, and minimize frustration. How big is this issue, compared to not being able to plug a pen drive, memory cards, or your phone to charge while your laptop is charging too? I now always travel without a dedicated phone charger because I use my laptop for that purpose, for example. I can change my habits,of course, but I'd rather not step back to two chargers in my bag (or add some sort of charging hub). This is particularly annoying when you already have to carry plug adapters for three different countries.
Additionally, the more I look at the beautiful Mac Book body, the more I think I'd have greatly liked the symmetry given by a second, identical, USB-C port on the right side.
Will USB C take off in general across the industry? Will we start seeing USB power supplies as a way to finally standardize the way we power our laptops, no matter who the vendor is?
My bet has been yes for a while. I almost give for granted laptops of any brand having one connector type only, but more than one.
I'd go as far as saying that we'll see laptops, maybe only best ones, that can be charged from any USB-C port, with some clever design solution to only allows charging from one at the time.
I hope I am not too optimistic.