Your logic works perfectly in a pre vaccine world where Person A and Person B have a roughly equal ability to catch, carry, and spread Covid.
That is no longer the case, once vaccinated we know you're less likely to catch covid, and if you do you are less likely to transmit it as well. There's a 94% reduction in transmission from asymptomatic carriers, for example.
Now, you can apply your logic on top of the % reductions that vaccines provide of course, but how far do we take that? Well it probably depends on the situation.
Seems to me there will be some scenarios there the additional protection to you and others of a mask will be of some benefit, over and above the herd being vaccinated.
So if you have a sniffle and are getting on public transport - please wear a mask. Maybe we should all wear them on public transport at peak times? Also other scenarios where it's illogical and done for the sake of being seen to do it.
Remember, what is a minor inconvenience to you and me may be more problematic to others. Some people need nonverbal cues. For some people a lack of masks makes them more anxious, or more confident. And for others the sight of masks on everyone makes them more anxious, or more confident. I think it's something we just have to tiptoe through and see how things go.