The convergence with power supplies for lower wattage devices is impressive. My iPad charger is 10W and just slightly smaller than this. This thing charges (and powers) an entire laptop).
I expect in a generation or so of iPhone & iPads, all of the devices will share a common charger capability.
iPhones & iPads will probably still come with lower-wattage chargers for cost reasons, but if you have an Apple laptop, it's very likely that you will be able to travel with a single charger for all of your devices.
It's also conceivable that a USB-C device may be able to charge at a lower rate with a smaller charger; charge your laptop with your iPhone charger in an emergency.
I expect in a generation or so of iPhone & iPads, all of the devices will share a common charger capability.
iPhones & iPads will probably still come with lower-wattage chargers for cost reasons, but if you have an Apple laptop, it's very likely that you will be able to travel with a single charger for all of your devices.
It's also conceivable that a USB-C device may be able to charge at a lower rate with a smaller charger; charge your laptop with your iPhone charger in an emergency.