We observe much worse acute and long term outcomes, across a variety of dimensions, when transgender people are not permitted to transition.
Homophobes insist that they are telling "the truth" when they insist that all gay people are going to hell and that marriage should not be allowed for gay people. Racists insist that they are telling "the truth" when they insist that black people are simply more violent than white people and that black people should be treated differently by the justice system. Sexists insist that they are telling "the truth' when they insist that women are not capable of holding positions of leadership in business or politics and that their role is only to raise children.
I see no reason why transphobia would be different.