If the increase is due to better diagnostics, then we should see a plateau after a period of time.

A few years ago, I remember hearing reactionary podcasters panicking about trans identity propagating by "social contagion" {their phrase}. The word "contagion" sets up an awful frame around this. I think instead you could come at this from an angle that recognises that there is a kind of social and internalised Overton window, a set of ideas that people are willing to express, or even allow themselves to think. As we increasingly see people living good full lives expressing and owning aspects of themselves that had once been heavily stigmatised - there may be a dynamic beyond just better diagnosis - there may be an uptick of people willing to be more honest about an aspect of themselves that they may otherwise have sought to hide or suppress, others might have been able to avoid even self-awareness. If that is the case - just as there will never be a person who matches the reference human genome - there may be no upper limit to the fraction of people who find some aspect of themselves that extends outside the venn diagram circle of neurotypical.

Quite - it's not as though sexual identity has always been a strict binary between male and female. Other cultures have explicitly recognised different gender identities for ages which makes me think that it's part of the Western Christian cultural identity to only recognise two genders. e.g. Thai culture has a number of different recognised sexual identities and of course, India has the Hijra.

To my mind, people who are anti-trans identities are simply trying to exert control over others rather than having a generous nature and accepting that different people are different.