For example, everything we associate with the magical age of 18 years old. "Adulthood", sexual consent, voting, drinking, smoking, conscription all begin on an arbitrary day and we take no consideration of the reality of the maturity of the person.
(Not to mention that "maturity" is entirely cultural and contextual. A harsher world (like one that most of humanity existed in for our entire history) causes earlier maturity. A gentler one (that we are obviously all trying to create) delays it.
So with animals, I see the spectrum, and I see that we are seeking to set a line...somewhere. Some set it just after humans. Humans are special, and everyone else is fair game. Vegans also have a pretty clear line.
Everything else, vegetarians included, are pretty "fuzzy". I'm an omnivore myself, but if you asked me for a concrete logical reason why my personal ethics allow me to eat pig but not dog, I don't think I could give you a ethically consistent one other than "pigs are delicious, and dogs are my friends".
Personally, I do believe that the same way that our generation asked our grandparents how they could've been ok with the racial discrimination & segregation that was ubiquitous in their youth, I think my grandchildren will ask me the same about my consumption of meat.
The reasons will likely be a mixture of climate impact (species of fish going extinct), and cost (because of the energy cost required to create pigs and cows). Society will form an ethical consensus about why it's not appropriate to eat animals to help reduce WW energy costs the same way that early people in the Levant found it helpful to create rules about not consuming pigs or shellfish (for sanitary reasons).
And there'll be "scandals" about rich people that have secret animal farms similar to what we have today about finding out about billionaires in dubai basically having secret house slaves. (AKA, outrage, but no meaningful change)
BTW from where I stand, it seems fairly reasonable to deduce that all mammals are in some way conscious/sentient, and have intelligence comparable to our own. Mammals play, mourn their dead, have a common signal that generally causes us to protect the young of all mammal species and find them cute.
I also think that there is a strong case for a very unique type of intelligence and conscience in Cephalopods. I have personally taken a stance to not eat cephalopods. Because I think they are in greater danger of going extinct vs. mammals (since their stock counts are so invisible to humanity), and I have decided (subjectively) that consuming a distinct type of intelligence from my own constitutes a form of crime. I don't know if I can defend that in court though.
I am not convinced about the rest of the animal kingdom.