I totally agree.I feel like pay it forward doesn’t have to be immediate or even the same exact thing. It’s a random act of kindness.
I'd go a step further and say that I think that simply paying for the next person's order is a lazy way of paying it forward and being done with it, and is basically refusing the kindness of the person who paid for yours. I would not participate in a chain of treat buying for people who were already planning to buy themselves a treat, but would absolutely plan to pay it forward at some point.
I do not need or want someone to pay for my coffee. I'd rather they give it to a charitable cause or help me out in my time of need. For example, if they saw me having a bad day, etc. The anonymity of the drive through line just sort of takes away from the act of kindness IMO.