It's just my opinion, but I think that we could consider two different types of time travel instead of the universe choosing whether a loop or a new timeline is created.
I think that if you use the time monolith to go back or forward in time, you are traveling within the same timeline, like a portal to another place in time in the same timeline. However, this doesn’t mean that you can change the past because the fact that you use it and what you do after using it, is already “written”. In other words, the journeys to other time moments via the time monolith have always been part of the same timeline from where you travel.
When Fitz used the Time Stream to search for the way to beat the Chronicoms, he saw that they needed Kora, but the only way to get her was to travel to another timeline and change it. That’s why I think that he and Simmons worked on another method of time traveling which doesn't involve a “circular” path within the same timeline, since that could not achieve the goal of getting Kora. They managed to develop a device capable of using the Quantum Realm (a kind of time travel that involves using other timelines, just what they needed). After that, they travel back to 2019 using the time monolith, something that doesn’t produce contradictions because they never inter-actuate with their past selves, and then, Simmons rescues the team and they all travel via the Quantum Realm to the 1931 of an alternate timeline.
But taking this as true, there's a problem with the two times the monolith has been used in the past (the one from 2018 to 2091 and the one back) because that future in which the Earth was destroyed was changed. But in this case, since these two usages of the monolith together create kind of a time loop and we've seen in the episode of the last AoS season where they are trapped in a time loop that sometimes there are anomalies (Daisy and Coulson remembering the previous loops) that give each loop the possibility of differing from the latter, I believe that in the AoS 5 situation, the random element in each loop was very little, but eventually concluded in avoiding the destruction of the Earth and erasing that future from existence (it only remains in the team's memory and in the form of Deke).
However, it bothers me that when they come back to the main timeline, they are actually arriving before the point in time when their past selves leave the timeline. The same thing that happens with old Cap. All of these could be explained using what @Marvelus has said before: the history of the entire Multiverse and how its timelines interact with each other is already written, as well as the differences between the loops and the transformation of a timeline like in AoS 5.
What do you think about this?
