The problem is we're linking it to appearances instead of maturity.

The problem with sexual relationships between adults and minors is two fold. First, the minor in question hasn't had time to fully develop the emotional intelligence to healthily and safely engage in a sexual relationship. Second, there is an innate power differential between a minor and an adult: usually the adult has means of supporting themselves, something akin to solidified social supports and experience, education and knowledge necessary to live without the day-to-day support of others. You put these together, and you have a relationship that, even with the absolute best of intentions, becomes inherently abusive. The adult holds all the power in the relationship, and the minor is left with no choice but to worship the ground they walk on, and worse, they have not developed the emotional intelligence to identify it.

The problem with these 2000 year old loli's is not their body; the problem is that they're often child-coded. They act like children. They do things that highlights their lack of knowledge and inexperience. What is often played off as a cute girl anime trope is in reality an indication that this is someone who you can conquer, dominate, and hold power over in a sexual relationship, and you can feel "good" about doing so, because you're, with the best of intentions, just helping them learn through your loving relationship. So what if you're fucking her while you do so. (/s on that last sentence just in case)

There is nothing wrong with finding petite women attractive. 30 year olds who look like teenagers are not a problem. Hell, as long as we're on the topic, I'll shock most people by arguing that admitting that a 16 year old has developed into an attractive and desirable person isn't even a problem, as long as you're doing so from a position of respect rather than intent. The issue is neglecting to recognize the power differential between you and that 16 year old, and convincing yourself that it's okay to engage in romantic and sexual acts with them while uttering deranged statements like "they're very mature for their age" or "I'm helping them learn and grow so it's okay".

Child coded characters are a problem, and hiding the magic number that supposidly discerns whether or not they're fuckable doesn't suddenly make things okay.