I'm pretty sure we're not going to get new art for all 135 Hoenn Pokémon. We got new art in FR/LG and HG/SS because the style was outdated, so new art was made to match the style that was established with the original R/S art. We got new art for the base form starters, but that's about it.
If you look on both the Japanese and English websites for OR/AS, they use the old art
everywhere. They didn't dare use any old art with HG/SS's website when it first went live.
I think this will more than prove my point:
Old art for starters and their evo's, even though the base starters have new art, they still use the old art here:
https://www.pokemonrubysapphire.com/en-us/pokemon/starter-pokemon/starter-pokemon
Old art for Groudon and Kyogre used alongside new art for Primal forms and Brendan and May:
https://www.pokemonrubysapphire.com/en-us/pokemon/legendary-pokemon/primal-groudon-and-primal-kyogre
The second slide picture shows the old art of the Hoenn starters alongside new art for Brendan and May against new Hoenn region art as a backdrop:
https://www.pokemonrubysapphire.com/en-us/
Towards the bottom we see the starter triangle, and
they're using old art:
https://www.pokemonrubysapphire.com/en-us/pokemon-basics/pokemon-basics/pokemon-basics
The only page I could find that actually shows the starter's new art is this one:
https://www.pokemonrubysapphire.com/en-us/story/story/story
As I've already mentioned, the old art still looks great and fits alongside new art, unlike the watercolor artworks that were used during Gen I and Gen II. When the HG/SS site was first put up, they didn't dare show a single watercolor artwork, they only showed new art. Here with OR/AS, they're using both new and old art.
While one could argue "oh he hasn't finished them yet", he made new art for Treecko, Torchic, and Mudkip, but they're only using the new art on one page. They're using the old art practically everywhere. Also take note how all of these pages became live
today, so I think it's safe to say that the old art is staying and will continue to be used in official media, guidebooks, etc.
I'm actually glad though, because I really like the original Hoenn art. While I love all of Ken Sugimori's works, I don't like a lot of his pose choices for his FR/LG and HG/SS art, I prefer the "generic" poses that are used when it's the first art they have, such as with the Sinnoh, Unova, and Kalos Pokémon.
Another thing is, in X/Y, the color textures of the Pokémon are directly taken from the Sugimori art. For instance, lots of people were like "why is Parasect's mushroom pinkish-purple", and that's because that's just how Sugimori colored it. If you notice, the new art for Treecko, Torchic, and Mudkip uses slightly richer colors, but from the screenshots of OR/AS, you can tell that the models have been unchanged from X/Y, and the colors of them match the original art.
