For ECNL?
Simple. A mtf athlete has an unfair advantage over a biological girl. Grace grants this. You seem to still be in denial over it.
Suppose I grant your claim that all past mtf soccer players were so-so when compared to biological girls.
This doesn't say anything about whether they received an unfair advantage. It just means that they were second rate before they got the advantage.
Given that we don't allow mediocre cis XY athletes to play down into the girl's league, we should not allow trans XY athletes to play down either.
Objection. The word "unfair" assumes facts not in evidence. In fact, it is the very nature of the debate we are having is what exactly is fairness and when does something cross into unfairness. I have even gone so far as to argue that there is no such thing as a fairness concept....much of it is in the eye of the beholder and there are only winners and losers.
Further, just because the bell curves shift doesn't mean any one athlete has an advantage over another. The flaw, again, in your model is you are regarding the trans athlete as a sub set of a cis male whereas the proper way to look at it, even from a starting base line, is as its own data set with its own distribution.