I actually have quite high hopes for the Battle Resort.
Yes, I am sad to see the Frontier go, but I think GameFreak knows we'd be disappointed as heck if it wasn't included. The Frontier was extremely popular, and one of the most hyped elements of the RS remakes. They would have to provide some pretty big justifications to take it out. There has been a postgame Battle Whatever in every Pokémon game since Crystal (that's almost a decade and a half ago), so I think they know how popular it is. A challenge sush as the Battle Whatever adds a lot to the longevity of the game, it gives you something to strive for, a reason to train your Pokémon to be the very best, and to explore the true depths of the battle mechanics. It's also a place to put the most expensive end-game items and TMs for players to aim for. They're not going to leave a Battle Whatever out just like that, and in the region of the original Frontier, they'll have no excuses not to deliver something epic.
But they might have wished to change/replace some of the old facilities, or adding new ones, which demanded so many changes to the Frontier that they decided to just make a new one from scratch. And while the Frontier looked awesome back in its days, I'm not sure on how well the facilities would scale to the 3D graphics. The place looked enormous on the little GameBoy screen, and you could run around for several minutes exploring new things there, but ultimately the place made great use of perspective magic to look more immense than it really was. In a top-down world this might have worked well, but it's not guaranteed to look equally well with new graphics.
However, new graphics allows for some pretty damn cool things. The Battle Resort sounds like an awesomely fun place, the name alone invokes images of days of fun in the sun. I'm picturing palm trees, beaches, bungalows and battle facilities. Maybe not the Olympic village feel of the Battle Frontier, but I'm expecting an equally good gameplay experience.