If it didn’t create a new timeline/alternate universe, then it changes the future in such a way that going back could never have happened in the first place, creating a paradox. So basically, if one can go back in time and is able to change history, the change has to create a new alternate universe, because if it didn’t it would break the universe.

The only other alternative is that going back in time can’t change anything because attempting to change things ends up causing the thing you want to change to happen (closed loop), like Terminator 1.

So if you can change history, you must create an alternate timeline/universe.

If you can’t change history, then no alternate timeline can be created.

Any other version of time travel makes no logical sense (insofar as any version of time travel can make logical sense), because any other version breaks causality.