There was no point in telling Sarah about her and Reese parenting John at all since it seems their plan all along was to travel to 1997 right away anyway in order to stop Skynet, at which point John's birth is moot. And, from T3, we know the trip to 1997 to stop the creation of Skynet would have been moot anyway since stopping it then only delays Judgement Day until 2003 and it can only happen with the discover of the CPU from the original T-800 Arnold Terminator, which they've already stopped and captured. So that right there would stop Skynet. Which is probably why the John Terminator (the T-5000 or whatever he is) had to go back in order to do Genisys. The actions have pretty much erased the possibility for Skynet to exist so something needed to happen in order *for* it to exist.
Looking over the movies and trying to "diagram" the timeline, which can only cause more confusion, leads me to believe that Skynet has some mastery or control over the timelines. It likely exists in the fourth or some higher dimension where it can see changes in the timelines occurring and can instantly make the necessary correction to ensure its survival.
It sees the failure of the original Arnold terminator so it sets into motion the T2 events (sending back another Terminator before the humans seize the time machine) the humans notice these two time-travels and send back a reclaimed T-800 from the storage space, reprogram it, and send it back.
T2 changes occur.
Skynet sees this from it's other dimension and sends back another Terminator to a later point, this time the T-X. Because Judgment Day was neutralized and pushed back everything else is shifted around as well, the rebellion winning in 2032 instead of 2029 now. Skynet is also successful in killing Connor in the future (probably similar to how we see it happen in TG) the rebels stop it, capture it, and send it back to protect Connor in 2003. As well as sending back the other Terminators to counter the other Terminators Skynet has sent back.
Now there's a question on what happened between T3 and the events of TG that "reset" the timeline, ignoring TS. Because TG takes place in 2029, the original success of the resistance.
Somewhere something else happened to change things to cause Skynet to send back a T-1000 to the 70s/80s to kill young Sarah and for the rebels to send back a T-800 to help her. This shuffles the dates around again putting J-Day in 1997 and the fall in 2029.
This also ends up changing things quite radically as it not only seems to undo the other movies and also causes a certain failure to necessitate *again* an even more advanced terminator to try and take John out in 2029, send him to 2015, and to ensure the creation of Genisys.
Really, the entire timeline continuity of this franchise is a hugemess and even this attempt at a "reboot" makes things confusing considering every change that is made seems to negate the possibility of anything ever happening. Especially since T2 more-or-less establishes the creation of Skynet to be part of a pre-destination paradox where the sending back of T-800, and its destruction, is the very thing that ensures Skynet's creation.