the story would've ended if it were that easy for ian to pack it up and leave with jo. the whole point of the story is literally ian STRUGGLING to leave
+bcs he's being tied down by tj/gang
and even if ian did leave with jo, in their current situation, it will be more difficult, ian is literally hunted down by various ppl who are involved in the gang he'll end up doing more harm to jo which he dont want. ian would've been happy if it was that easy to have a normal lifr and a new beginning, since thats literally what he have been wanting from the start.
I like TJ as a character more than the other 2, but I still wouldn't want him back together with Ian unless they're truly both done with the gang life and TJ isn't gonna be for a while (if ever). He chose this way to make it up to Ian and to still hold something akin to guilt over him so he won't forget him that easily.
As for Jo, dude's becoming a mess and getting involved in stuff that's way over his head for a "love" that's not much more than lust and curiosity at this point. Also, if Ian rly cared for him, he would've packed up and left together with him already to have a fresh start somewhere safe for both. This relationship of theirs is far from a priority to him though and he's subconsciously using him to either feel normal or to fill the gap that TJ left and I actually wish he'd leave Jo be instead of dragging him further down.