Person-first language was popularized by parents who want to believe there is an allistic version of their child somewhere. That eventually they can “unlock”* this version of their child and they can drop autism like it’s a purse.
In truth, autism is intrinsically linked to who their child is. Erase the autism, you’re left with a completely different person. That is why the autistic community overwhelmingly prefers identity-first language. We understand that autism makes us who we are and that’s okay. It has its benefits and downsides but we are never going to be allistic and we should accept ourselves the way we are.
*Seriously, I’ve seen parents use that phrase and I thought they were talking about their children like they were video game achievements.