I guess they wanted to conform to the USB-C standard and that that meant not having a magnet.
The MBA was already too light for MagSafe 2 in that they had to make the magnet for it so weak for it to be safe that the cable fell out all the time by accident. I'd still prefer a slightly overpowered magnet over no magnet at all, though.
Think outside the box. You don't need to have the USB-C itself break away. Just have a USB-C stub with a MagSafe just at he end of the stub. If the cord is pulled the MagSafe breaks away leaving the stub in the MacBook.
Since the USB-C is standard, and the other end of the cable is regular USB, I imagine that several companies are building a replacement cable incorporating this idea. It won't be Apple's MagSafe because of patent reasons but it can be a similar, magnetically-attached device.
----------
The iPhone doesn't have MagSafe (maybe it should?). Perhaps the thinking is that since it has all day battery life, it will only need to be plugged in overnight when not in use.
The iPhone is so light that MagSafe would have a hard time working. In addition, MagSafe terminals are exposed but you probably don't want the data connections to be. That could be solved but would need isolation work.
----------
Yeah, good point. I honestly forgot Google included a Type C. I stand corrected.
And the Google connection thingies should work with MacBook !
First time Apple and Google have worked together for ages