Ok… So let’s play this out
And what do you suppose happens when the Democrats see that when they adopt transphobic positions, people who support trans rights will still vote for them, and maybe they pick up a handful of transphobes? In that case, there is literally no downside in that situation (strategically speaking) to adopting transphobic positions all across the country. If on the other hand, people draw a red line on trans rights and stand by it, if every transphobic Dem eats shit, then there's a clear strategic benefit to respecting that red line.
You are only considering the immediate effects, but the precedents that we set are far more important. If not getting a majority this time is the cost of stopping the party from shifting right on this, then the benefits outweigh the costs.