Rights based discourse always has a flipside, the not-havers of rights. Doing away with souls and rights and switching to a frame of welfare would be a pretty huge boon for animals as well as many humans.
The source for alleged rights is always some scripture, some arcane interpretation of nature or history. You can always deny people their rights based on some text they allegedly derive from. You can, by definition, not deny people their demand for welfare or subjective interests.
Case in point, the right to vote. Not only didn't it magically extend to everyone because it was conceived of as a right, arguably the opposite was the case.
It's not romantic (you might call it deeply cynical) or all that satisfying, emotionally, but it's extremely parismonous and cuts through many moral dilemmas.
It's "social contract": I grant you personhood because you've forced me to. How? Because you can kill me if I'm mean to you. And you can really help me if I'm nice to you. By treating you like a "person", I'm extending a set of manners to you. I do this because you can reciprocate a social contract with me, both negatively (harm me if I harm you) and positively (help me if I help you) to a great enough extent.
Even if I kill you because you're powerless against me, other people will know I killed you, and they might kill me because they're scared I'll kill them, and people have written this down into laws saying you don't have the right to kill others, and so I have to resist killing you because you are "a person". Maybe I'll also feel bad because I'm programmed with empathy, which is another way you fill the social contract (I'm harmed by harming you).
What about a dog? Or a person in a coma?
Here it gets circular. These beings cannot fulfill the social contract to the extent a normally-functioning adult human can. But if normally-functioning humans care enough about these beings to extend them personhood, I'm again forced to extend personhood to those beings as well: if I don't, the more powerful humans will harm me. Or, to flip it, if I [harm|help] beings that the powerful humans care about, I'm [harm|help]ing the powerful humans. Because they're powerful, they will reciprocate the social contract in kind.
The circularity is: if enough humans decide a tree is a person[0] and will charge you for murder if you kill the tree, you have no choice but to go with it.
Basically: you go along to get along, unless you don't have to and can force others to. That's always the way it's worked and will work.
0: https://www.fastcompany.com/90738287/this-tree-owns-itself-a...