They traded a causal loop paradox for a grandfather paradox. Paradoxes are illogical by definition.https://en.wikipedia.org/wiki/Temporal_paradox
Most TV shows expect you to either accept the paradoxes or to just not think about it. They can't all be SG1, where the time travel rules are both consistent AND make sense. I'll settle for consistent, which will depend on if and how they revisit time travel in future episodes.
Just going to drop this here. lol. I did 6 parts and two were dedicated to just paradoxes and loops. They were the strongest of the series in my opinion. https://www.feartheboot.com/ftb/index.php/archives/1238https://www.feartheboot.com/ftb/index.php/archives/1240
So for a full logical evaluation of this time travel loop I would need a whiteboard. Robin is what makes this all come together. Basically each of these are alternate realities being split from the original:
Timeline 1: No one was pulled out of the past and the future happened. Robin's visions led them to create the time travel machine that would bring Shield there. They then pulled shield from the past. When they went back it created timeline 2.
Timeline 2: Shield was pulled out of the past. They interacted with the future of timeline 1 and were then send back. Minor changes likely occurred here over their original actions, but not enough to prevent the future. This is the point the loop was created. This loop could have occurred any number of times.
Timeline 3: Shield was pulled out of the past. They experienced the future of Timeline 2 or one like it. They knew that they were in a loop. When they went back they created timeline 4.
Timeline 4: Shield returned from Timeline 3s future. They learned enough from that future to make a few key choices and prevent it from happening. This created a new timeline where that future never happened. The Fitz in space is now in this timeline and the dystopian future they saw is locked away from him. He is no longer needed to fix it.
Now the question is what happened to the other timelines. They are all alternate realities, but do they exist out in the multiverse? If this is true than you never truly change time but just create alternate realities. Your reality will always go on but you will just be locked away from it in a new reality if you make a change.
If instead each reality exists only once and time travel actually changes it then the old realities cease to exist when a change is made. If Zeke disappeared by fading away that would indicate this to be the case They were vague on how he disappeared though and he might have just left to come back next season.
Alternate realities are the ultimate answer to paradoxes.