Starting out in MTG, but core sets are being phased out, so what to do?
If you want to start with a core set, Magic Origins is fine to buy - it's going to be in standard another 18 months. The changes are all in the future: no more core sets after this one, two-set blocks, changed rotation schedule. But that doesn't change that Magic Origins is here now!

Even if nothing were changing, it'd still be another year until the next core set, and if you're going to actually get into the game, a year is plenty of time that by then you wouldn't be worrying about core set vs "expert" expansion anyway. So it doesn't seem like the changes are really taking any intro-level stuff away from you. Besides, Wizards knows they need to keep the game at least a bit accessible to new players; I'm sure they'll keep creating things like the Duel Decks that'll give you a way to pick it up even once the core sets are gone.