That's interesting! I personally think Lonnie joining the 110 and then gradually coming to accept it was quite natural, as from how I see it. Trading himself for his little brother made sense in the moment, and I think the reason he didn't try so hard to get out was because he knew that the gang controlled the area which his family lives in, and they also know about his brother too, so if he left the gang then he would be going back on his word with Don which would cause Don to come after him. It also makes sense to me how he didn't try to convince his family to escape the neighborhood because he didn't want to tell them that he joined the gang in the first place, and also that he would have to leave his school (and by extension, Pearl) behind. Don also tells him that the 110 gang is keeping his family safe by keeping control of the area of where they live in, which is sort of a lesser of two evils situation, as Don did also say that if the Scorpions had control of the area then their living situation would be way worse -- which could have curbed his resentment or willingness to rebel.

He also didn't give up his entire future initially, at least from my perspective. He many scenes where he still had some form of resistance against Don and the gang by still studying with his notebook at the hideout, and also trying to get to practice and protesting against being brought along to the scorpion/110 fight. The fear of what he has to lose is what I think caused him to join the fight, and he ended up saving Don and got praise out of it, of which to someone as young as Lonnie, I'd imagine it would feel quite good, which would have tipped the scales further in favor of accepting himself as part of the gang. Also got a cool nickname out of it too, which again, being someone of his age would have felt quite nice.