When it comes to the participation argument, you gotta bear in mind that there are also people (FTM) moving the other way. From a macro overall perspective, the numbers should near balance out. You are still dealing with the same overall numbers in each pool. The question is when you assign each to the other, and how you divide up scare resources within the pool such as scholarship money. It puts folks in the unseemly place of having to argue "don't touch my scholarship money", which is why they rather argue about fairness or safety, but as has been shown, neither gets you there 100%.
Has a trans girl ever in the history of planet earth ever taken away scholarship money from a cis girl? Or is it just fear mongering by transphobes who are making up unrealistic fake hypotheticals because admitting the real root cause of their position (which is that they just hate trans people or at best don't care how badly they're abused in life) isn't exactly a flattering argument?