At the end of the day the Theo storyline is just the latest symptom of a much larger underlying problem. For a show that is ostensibly about the importance of found/chosen family, 911 has an unfortunate and consistent pattern of showing viewers that the opposite is true, that blood is always thicker than water.
Nia goes back to her bio mom. Mara gets to stay with Henren but only because her bio parents are dead- found family is fine, as long as there's no bio parent backup. Denny has to forge a relationship with his biodad, and Henren have to allow it despite the man acting innapropriately, because he's the biodad and blood is always thicker than water. Chim has to form a relationship with estranged brother and abusive father because they're family. Buck has to forgive his neglectful parents because they're family. Eddie can't cut off his parents because they're family. It doesn't matter whether they act like family or not, because they're bio related, the show says they have to be important. They have to be forgiven. They have to take precedence in a characters life over any 'family' that isn't blood related. Blood is always thicker than water.
Why is Buck the best choice for Theo if his parents are out of the picture? Because he's the bio dad, and in 911 universe that's the trump card. It doesn't matter that he just met Theo and that Theo just met him, and that Theo almost certainly has other loved ones who would be better suited to take him in. Because Buck is the biodad, the show says he's automatically the best choice. Blood is always going to be thicker than water. It doesn't matter that Buck already has a pseudoson in Chris. Theo is biologically related to him and Chris isn't, so now Theo is always going to take precedence. The show makes it clear: found family is fine but only until "real" bio family shows up. In the end, blood is always going to be thicker than water.
It's the unfortunate curse of the show that means Buck is always going to take Maddie's side over Hen's, and that Chim will miss Karen's birthday to help Albert move. It's why Jee-Yun and Theo will get to be cousins but Jee and Chris never were, and why if Eddie's sisters show up his relationships with Hen and Chimney will shrink even more.
In 911, blood is always going to be thicker than water. Found family will never be given the same weight as 'real' family, and if you ask me that's the biggest tragedy of the whole show.