Fair point. But this isn't a get out of jail free payment, you're not "offsetting" your cup, just getting charged for it. The social stigma of walking around with a single use coffee cup remains, and may even be amplified. Not to mention that "cafe coffee", at $4-5 / cup (in Australia at least), is already very much a luxury item.

The cup is a product that is currently perceived as gratis / of zero value. Charging for it is intended as a disincentive. In the case of plastic bags (here) it largely worked as such (with prices as low as 10c per bag!) and paved the way for a ban. Though the parallel effect of more resource-intensive bags / cups entering the market / being treated as single use is always a consideration.

But to your broader point, for sure .. bring on a ban.