In-universe, most series don't make a ton of sense since Duel Links is a virtual world created by Seto Kaiba.
I wouldn't look too much into it. DSoD is likely happening to delay the power creep a new Extra Deck card type would bring with it, and give Konami more of a chance to introduce DM/GX/5D's characters without every second dumbass asking "why isn't this a Zexal character?"
Technically DSOD came even before too GX. since dsod happen like 6 month later of DM ending. of course the movie is based on the manga, so nothing related to anime.
It makes a lot of sense to delay zexal. Synchros have not done well till date and there is no good reason to introduce a new type of card. Arguably the only synchro deck that works as a standalone deck but can up the stakes by synchro summoning is six samurai. All the other decks either need to be planned with synchros in mind or you need to throw in some tuners just to get synchros out.
Technically, Synchros not doing well is Konami's own fault.
They didn't gave us what's needed to make Synchro dominates the meta. Trident Warrior + Delta Flyer combo is a step in the right direction, but not enough. We also need good supports for the Synchro itself once it is summoned.
They have to be really careful about what synchro support they give us. A few wrong cards added, and we could face the same fate the TCG went through for years of obnoxious loops, FTKs, insane 10 minute combos, etc
I wouldn't write off a card's brokenness potential from being a coin flip. There's been multiple broken die-roll cards (Snipe Hunter and Sixth Sense), so it's not out of the realm of possibility for a coin flip card to be broken.
6th sense draws you absurdly amount of cards or graveyard setup for free. Comparing this to that is like saying Dark World Dealings is on the same plane as Graceful Charity