[CydeWeys]

Defense in depth is a useful concept for a lot more than just security.

Yes, of course people shouldn't litter. But some will anyway, and even in the unlikely event that no one on Earth litters ever again, plenty of trash will still escape from trash cans, trash trucks, etc., especially on windy days.

So the answer is we need defense in depth against litter. Littering should of course be curtailed to the maximum extent possible, but the litter itself should not last indefinitely upon escaping into the environment. It should degrade in a reasonable amount of time, or at least be totally inert and non-disruptive to animals and the environment (which plastic is not).

So you're both right. All available measures should be taken, so that combined, the optimal result is achieved.