^I will elaborate on that completely when I finish my blog. But well I can take some time hehe, while the characters' phrasing allude to rewriting, it doesn't make sense in Runaways.
*Chase's Time Machine works creating closed timelike curves, which would be the real life result of an equation that would make time travel possible. A closed timelike curve is based on how strong gravitational fields would create a "black hole" in the space-time continuum, which would force the past to meet the future. You would create donut shaped vacuum and each turn you make within it would take further into the past. That is why Alex had nauseas.
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*Now, the thing with a CTCs is that by using it you would return to exact same point you exited from. So if I create a CTC in 2028 to go to 2021 when I return I would return to my 2028. Real life physicists say that to explain a CTC existence and how time cannot be rewritten you would have to come up with the predestination paradox. So the time-traveller was always part of the past and you can't rewrite anything.
*Stephen Hawking thus proposed his chronology protection conjecture, he says time is relative, and therefore if you manage to time travel your actions would be meaningless on a universal scale and you won't be rewriting because it is impossible and all the changes you could have made in the past are already "done". You can't kill Hitler before 1945, because we know Hitler died in 1945, and therefore you won't change anything. Chase can't save Gert in 2018, because she died in 2018, his 2018 at least.
*That is why the CTCs allign with David Deustch's proposition of multi-world (yes, Deustch is the one mentioned by Iron Man in Endgame). So if you use a CTC to go to the past to kill Hitler as a baby, and if you kill him then you would be doing it in an alternate universe, not the one you came from because from where you came Hitler died in 1945.
*There is a CTC interpretation that if you kill Hitler in 1945 you would have created a new timeline, but when yo go to your present, you would realize you didn't change your history. The Avengers explore this concept, but they do not use a CTC they use the Quantum Realm, which instead of making past and the future meet, it just take you out of your universe from a certain time period, and reenters you in your universe in a new given date.
In the MCU predestination paradox and branching happen when you deal with time travel. I am just explaining Runaways here.
*If they were rewriting, then Alex (2028) would have ceased to exist if he had killed the Runaways in 2021. Because if he had killed the Runaways in 2021 then the reasons of why he decided to kill the team in the past would non exist. Let me explain this better.
**A member of the Runaways tries to kill Alex, the member fails and Alex in 2028 decides to kill them all because he does not know the member.
**However, if Alex 2028 killed all the Runaways members in 2021 he would have erased himself from existence, because if that Runaways member died, then that member would have never tried to kill Alex, and therefore none Alex 2028 would have traveled to the past. So, it is not rewriting.
*Alex is just trying to kill them in out of despise. It is similar to what the Chronicoms believed they were doing, but they were just creating an alternate universe with no Runaways in and no S.H.I.E.L.D. respectively.
*When they go in time to 2017 the day before the pilot, we see that Gert Yorkes reacts abruptly to Chase Stein, she doesn't like to be around him, but when Chase 2028 says nice things about her (remember the Pirates of Penance play, I think that is the name), this greatly affected her and she started to see him differently as her crush. And that is why the next day, Gert has feeling about him and feels disappointed when he does not attend the meeting at the coffe house. Chase 2028 is the reason why Gert 2017 fell deeply in love with Chase 2017.
*This is confirmed when the team then time-travels to 2018, because Gert 2018 identifies Chase 2028 as "Pirates of Penance". The reason of why 2021 Runaways, and 2028 Alex and Chase disappear is related to the act of saving Gert Yorkes. However, they are not rewriting.
*Alex and Chase created a 2021 reality when they went to the past. The team did not create a new timeline in 2017 (ripples) but they did in 2018 with the actions they made. In the comics, in Uncanny Inhumans Vol 1 Issue 2-4 we see how Kang time travels to the past and starts to change the Inhumans history, which results in Gorgon and other Inhumans disappearing in the present day, and Beast says something key: "they are being erased because they do not belong in this new history".
The Time Stone controls and represents the escense of time, and thus it describes what are the multiple consequences of manipulating it.
*So by using a CTCs you would get erased from existence (or just abruptly taken out) if you jump to a timeline you cannot exist in. (Mordo warns temporal manipulation can erase yourself from existence).
*By using the Quantum Realm if you do not take special care, you would have time going through you and deaging you or further aging you (this is what the Time Stone also describes, remember the apple and Hong Kong)
*By using the White Monolith properties, if you don't control it properly you would be stuck reliving the same events over and over again (a loop) this is just again what Mordo warns Strange about messing with time.
In the MCU, you have predestination paradox if you do not change the course of history RIPPLES (Wilfred delivering the serum, Barton in 2018, Sousa not dying in 1955, and the Runaways 2021 always being part of 2017), but you would create branched off timelines if you alter the course of history (Luke deciding to stay in 1955 and protect Malick's survival, Coulson slipping in the serum, the Avengers 2023 arriving in 2012, the Runaways saving Gert Yorkes)
