>> One of the reasons is, half their cells are using one X chromosome and half are using the other, so they're more likely to get their performance dragged down by deleterious mutations.
That sounds like something has been severely misunderstood by someone, possibly even me. Are you a biologist?
Yes, I'm sure they were brought up proper and are very honest, but in order to understand them you need the relevant training. Looking at your profile I can't see that you have that sort of background. So I don't think your confidence that you know what's going on is justified.
Or, like the saying goes- a little knowledge is a dangerous thing.
That sounds like something has been severely misunderstood by someone, possibly even me. Are you a biologist?