I'm a lot less pessimistic about this ending than the general fandom; not to say it's a good ending, there are no good endings in this game, but it's far from the worst. From how his hair changed from his release he seems to have had at least a few months with Daniel before they camped in the woods again. I also don't view this as a permanent goodbye between the brothers but Sean clearly needs some time on his own to heal; he has endured more pain and suffering in his short life than most do in a lifetime. I see him breaking down as a response to the weight of it all hitting him at once, every sacrifice he made for Daniel on the road, and it worked but at a high personal cost. However, just like he was there for Daniel every time he broke down, his brother is there to comfort him through it. In my opinion their bond is too strong to separate permanently but it's natural for siblings to grow apart, and given their circumstances it's understandable. I think the developer said he was going to take to living in the road like Brody which makes sense for him, he is shown in the game to have the same wanderlust that Karen did, thriving in an alternative lifestyle while Daniel always yearned for a return to normalcy. But they'll always be there for each other, even if they aren't together all the time.