That's a pathetic argument. Nothing ever happens - until it does. The first athletes to use steroids and other PEDs were not an issue, until a few started setting records and winning championships. Then in a very short time frame it became a widespread scandal.

Transsexuals, until just very recently, were in the closet, so of course it hasn't happened much yet. But the cases are increasing, as shown in this article, with a female wrestler, and there are undoubtedly more cases that we haven't heard yet, like a friend of mine whose daughter was crushed and injured in a soccer game by a beast of a trans woman - she ended up quitting soccer. With more and more transsexuals coming forward, and after seeing success for trans women athletes, it will naturally lead to more trans women competing. And your argument of "hasn't happened yet" will be null and void.