By definition, if the 1984 timeline has time travel in it, it's NOT the original timeline anymore.
1984 was just going along minding its own business when two time travelers appeared. What part of that makes it not the "original" timeline? Looking at it another way, if your definition of "original timeline" is "one where there is no time travel" then there was
no original. Closed time loops don't have a start... that's part of their definition. They don't have a first iteration and they don't have a last one. The just are.
One of the things with time travel is the effect preceeds the cause. Just because the cause happened in the future doesn't mean that there ever had to be a 1984 where the effect didn't happen. In the first movie, they didn't have a ripple effect or alternate timelines or anything... whatever happened, happened. And always happened. And always will happen. Going back in 2018 or whenever didn't change the past because that was already part of established events.