Why don't more things change back?

I think it's just because the "loop" in which Joe kills himself is that final loop, and something like killing someone is irreversible. But who knows, another Looper that knows Joe could come back and kill Joe before he kills that boy, and so in that loop the boy will live.

Basically there are many loops with many different versions of our world, depending on certain events. Since time travel isn't real, the director and writers can do whatever they want to do like the writing on your arm, will suddenly be engraved on the future version of yourself. In actuality, if time travel were real, the carving on the arm would have always been there, it wouldn't just magically appear as you're writing it. But this is up for debate as well. It just makes the most sense to me.

I've been thinking about this for a while and I think it's best explained by saying time isn't continuous but has lots of branches.

At the end, when young Joe kills himself, old Joe ceases to exist in that reality although what has already happened cannot be undone.

There will be another timeline in which young joe never existed to come back as old Joe to be killed be himself. This reality would create a brand new loop where young Joe wouldn't have killed himself (because Old Joe never existed). He would have then grown into old Joe again but because the rainmaker never existed, there would be a whole new reality for him. Maybe he gets sent back to get his loop closed, maybe he doesn't, I don't know if loop closing was specific to the rainmakers reign.

Just my thoughts.

In the Looper universe, the "change" asked about only happens to people who are visiting the present from the future. And the only type of change that happens is that the people from the future will bear the effects of anything that happens to their present-day selves.

For example, cutting present-day Joe will result in Old Joe's body suddenly showing a scar -- but it won't change the impact of any actions Old Joe has already done in the present.

The movie itself provides two exhibits for not changing more than purging old Joe from existence. After young Joe shoots himself:

• Cid still has a scar,
• The car/truck whereby old Joe had arrived is still on the road.

The reason is that everything happend in the past is definite.

Its called time paradoxes (see my own grandfather clause) which can real hassle to understand. Many shows have done content about time travel/time paradoxes/abusing said paradoxes to one benefit. The writers have considered a lot of the stuff but, due time and or script, stuff got left out that would better help people who are not familiar with time paradoxes (thus for the better or worse we got what we got.)