When you ask what it's like if everyone were to "do that", the answer you get is going to be determined by how you define "that". For instance, if everyone were to drop chocolate wrappers on the lawn of your annoying neighbor, you might be happy. So is it okay to drop the wrapper on your neighbor's lawn?

I do need to be careful to define "that" as a generally applicable rule. In this case, the generally applicable rule would be, is it okay to drop chocolate wrappers on the lawn of people one finds annoying?

So I need to consider the world in which everyone drops chocolate wrappers on the lawn of people they find annoying. Considering this, the chances of someone dropping a wrapper on my lawn becomes dependent on the probability that someone will find me annoying.

So, in short, I can put as many qualifiers on the rule as I like. However, I have to be careful to attach my qualifiers to the true reason for my formulation of the rule; I cannot select the rule "it is acceptable to drop chocolate wrappers on that exact specific lawn over there" without referencing the process by which I chose that exact specific lawn.

I can't attach a qualifier to a specific person; but I can attach a qualifier to a specific quality, like being annoying, when considering a proposal.