Okay. Here's my train of thought: Barry-2024 already lost Iris and, because of that, everyone else. He has no onenot even villainsand probably goes crazy in solitude, obsessing over that moment when Savitar killed Iris and how to fix it. Problem is, he's too slow.
What we learned last night from the new girl whose name I don't care to remember, though, is that Savitar's suit actually decreases Speed Force friction or some such, which is what allows Savitar to be way faster than regular speedsters.
So, Barry-2024, in his obsession to go ever faster to be fast enough to stop Savitar eventually realizes that the only way he could possibly get that fast is to actually become Savitar, and the only way to do that is to fuck with a past version of himself to ensure that his past self will follow his own path.
The only (huge) problem with this, of course, is that Savitar already experienced a version of the present that's "out of order," compared to Barry-2024's timeline, and regular Barry already knows that Savitar is future-him. The only ways to reconcile this cleanly are to either say that Barry-2024 knew all along and was just lying to Barry, or they could just say it's an alternate timeline.
I'm leaning towards alternate timeline, as otherwise, there's really no feasible way to have Barry to save Iris and beat Savitar without also creating an alternate timeline, plus we already know that alternate timelines exist in this universe.