Wattage is just a measure of how much potential is available. If your laptop only uses 50 watts, you could plug at 5000 watt PS in, the laptop will still only draw 50 watts, and the remaining 4950 watts of potential is just that, remaining potential sitting idle in the PS. So you simply build the universal PS to output whatever the highest wattage requirement is. Won't hurt anything.
The problem is, in their attempt to cut down in e-waste, they are overlooking that they will create electrical waste. Most cellphones operate at the same basic voltage and wattage. Laptops don't. Yes, one universal PS with a maximum wattage will work, but for all the people using laptops with lower wattage requirements, there is extra energy being used by the PS for wasted potential that will never be used by them. Which is more damaging in the long term, wasting energy or e-waste?