I believe the addition of the Frozen cast was a huge money grab, but to say it brought OUAT back is not on the mark. OUAT was going strong without Frozen. Frozen was just a huge ratings grab. There woul still have been a season 4 without frozen and the creators of the show even admitted to the appearance of Elsa being a last minute decision.
Why would that have anything to do with the show Once Upon a Time? If the show was going to go on, they already had it planned. If it was going to end, they already had it planned. There are so many details and plans that go into these things, they can't just make changes of that proportion on a whim.
No I do not think that the popularity of frozen provoked the return of once upon a time. Once upon a time was a very popular show even before they introduced the character of frozen into the picture. I am pretty sure it was scheduled to return and not provoked.
I think Once Upon a Time would have come back without the popularity of Frozen. While the tie-in with the popular movie could certainly increase ratings, I think the show has had a pretty loyal following and has done well for ABC. There are enough fairy tales that people love to continue the show without the addition of the Frozen characters.
While I believe some movies have enough popularity that they spur other films or franchises it doesn't mean that was the case with Frozen and Once Upon a Time. Frozen didn't come out that long ago and it's popularity was spurred by the overwhelming amount of commercials/ads/marketing that went our right before it was released. The success of Frozen was created through marketing not by real human desire.