I'm afraid that, given enough generations, transgenderism will once again be considered a mental illness. It generally takes a child to see that the emperor has no clothes.
A sign of mental illness is having delusion, for example thinking you are Napoleon. A man who thinks he is really a woman despite physical evidence to the contrary is suffering from a mental illness.
All I'll say to this is that there is a lot of information on this topic that a large part of the population isn't aware of. For example, the concepts of gender expression, identity, and body. There's plenty more to think about and learn, like how this statement above might breakdown when you start talking about someone who is intersex.
That's a very tough question to answer because the two topics aren't as rigorously researched. Quick google searches seem to say around 0.5% of the population for intersex and 0.3% of the population for transgender. But again, those figures are probably problematic and shouldn't be taken as fact.
