Then, where was the "one day younger Barry" during the whole episode? And what are the writers going to do with him from now on? Right now there should be 2 Barrys in Central City, and they both would exist forever because the younger one is not going to travel to the past after the timeline has been already changed, everything during that day has been different, if everything had been the same, then the younger Barry would eventually travel to the past causing a time loop, and the "one day older" Barry could go on with his life, but right now we should have 2 Barrys in the show, this is why the writers decided that the Barry from the past had to disappear, but they just didn't explain why and it contradicts what we've seen in previous episodes where travelling to the past didn't cause the vanishing of the time traveller's younger self (old Barry and kid Barry the night his mom was killed, none of them disappeared, the older one just returned to the future, I guess).
Exactly.
Sorry, but that makes no sense, there was an older Barry trying to save his mom's life and the younger Barry was still there, he is the Barry we know right now, none of the 2 Barrys (old Flash and kid) disappeared after a new timeline was created, otherwise the Barry from the show wouldn't exist and we would have a much older Barry as the main protagonist.
Anyway, if changing the timeline caused the vanishing of one of the two Barrys, the one that should vanish would be the "one day older" from the old timeline that was erased, the one with the memories of everything that happened during that day in the previous and erased timeline, but that's not what is shown in the show, and if that happened everytime Barry travels to the past, like I said before then the Barry from the show wouldn't exist because he would have vanished when he was a kid and his mom died while an older version of himself tried to save her.
Travelling to the past doesn't erase your younger self, that was established in previous episodes, and time travel rules must be consistent, the writers shouldn't change them depending of what they need for each episode, and if they do it they should explain why the rules have changed, otherwise they would be awful writers.
I explained all of this (among othe things) in my previous message:
http://forums.superherohype.com/showpost.php?p=30955083&postcount=167
If the writers don't explain this, it would be one of the biggest writing mistakes I've seen in my whole life.