Here's my take. I'm not going to try to convince you that you need to wear a mask to avoid a breakthrough infection, given your vaccination status. You're right, your risk is very low. And even if you are infected, it will almost certainly be mild. So I agree with you, that's not a good argument. In fact, before Delta I would probably have agreed with you a lot more than disagreed.
The core reasons for me that mask mandates are still necessary for now have to do with three facts:
* Vaccinated people who catch COVID can still spread it, although at a much lower rate than unvaccinated.
* There's a sizeable segment of the population who still cannot be vaccinated - kids. And yes, kids aren't as impacted by COVID as adults, but because Delta is so transmissible, a large number of kids are ending up hospitalized - so much so that some pediatric hospitals in the south have been overwhelmed and not able to support non-COVID pediatric needs.
* The level of vaccination is sufficiently low that there's the potential for breeding of variants that could potentially break through the vaccines. Now, the obvious solution would be to require vaccinations in public places, but since that's been ruled out legally, mask mandates are all that remain.
Of these I personally see #2 as the most compelling. Given the pediatric hospital situation with Delta in high community spread areas, wearing a mask seems prudent to help avoid that, until such a time as kids can be vaccinated.. 5-11 year olds should be able to in a matter of weeks, and 6 mos to 4 years by the end of 2021.
I'd still prefer a higher vaccination rate to end masking, but I don't know how realistic that is at this point. I could be persuaded to end the mask mandates once the pediatric hospital situation is under control and kids have an opportunity to be vaccinated - so we are talking probably another 3.5 months (assuming all kids are eligible by EOY and allowing a month to get them vaccinated if they want.)