You're totally right that reducing waste daily is huge. My old office had the same cup pile-up until I brought in a cute reusable mug and just kept using it. A few people asked about it, and now there's a whole shelf of personal mugs by the coffee machine. @max41, it might start as just some cups, but seeing that physical stack every day really shows the scale of the problem. Cutting that out feels more direct than an offset you never see. It's a simple change that actually stops waste before it happens.
Yeah, it adds up fast though. My last job went through boxes of those cups every week. Once you start counting them all, the number gets pretty shocking. It's not a mountain, but it's a steady stream of trash that just didn't need to be made.