shmot, peach v lucas is a matchup that I have a lot of experience in as well. lucas's fair hits earlier (though not quite fast enough to do on reaction to seeing her start a fair), and has more range when telegraphed fully. Additionally, you can throw off the peaches sense of timing on the fair by training her to attempt to hit a retreating target and then FH fairing at her to get her to fair late.
The reason FH fair is important in the peach matchup is not because of her fair though, it's because of her dair. Lucas cannot beat it outright. He NEEDS to work around it, which means if she's already started her dair and is in the correct place, he needs to turtle or retreat to avoid the inevitable combo. If he can predict it, FH fair wins.
But overall, you're mostly right. If I see a peach fair headed my way, I'm not gonna try to beat it with my fair unless I know when and where it's coming from ahead of time. I'll retreat with a PK fire on reaction. That doesn't change the fact that fair is important against those who come from the air, including peach, for other reasons.