Until new updates with new features are coming out. Then you have to texture all those features for yourself in the old style, or accept that the textures of most new features will clash qua style with your old ones.
Because from the moment the new textures are implemented, every single new feature, from mobs to blocks, will be textured in the new style.
This is true, but what I can tell is that the style is not changing all that much rather it is just getting a face lift. This might just be me, but this still feels like Minecraft, just like how Faithfulx32 still feels like Minecraft and how Vanilla Tweaks still feels like Minecraft. If you want to retain the textures of 1.12 past then you can go about this via my method, but to be honest Minecraft never really had a singular style in my opinion. From what @xDadiKx1 is suggesting doubles up the work Mojang has by making them work on two separate resource packs for the game. If people want to maintain the "old style", by means they can. People just need to understand that what is Vanilla will always be Vanilla because Mojang made it and no one else has. Things are allowed to change and people should be open minded about it, but Mojang certainly didn't throw these texture changes at us with out a way to retain what the old minecraft felt like. People can also play older versions of the game if they hate the texture changes all that much.
But to actually be implemented to the game, I don't like downloading new resource packs just to give it back the original feel.
honestly though, that is REALLY insignificant to something you can simply add yourself by simply extracting the resources already downloaded on your computer from the Minecraft jar. You dont need to download a resource pack anywhere. You just need to select a version of minecraft with the old textures and extract them and make a resource pack out of it.
I've been around with Minecraft for a very long time, almost since when it first came out. Honestly these changes interest me and I respect that they are changing up the textures a bit. People need to start embracing change because change can be good. We still have yet to see a final product, and considering people know Minecraft as it is right now, it is hard to judge the look and feel of the final product since we are so familiar with the old textures. Once all the textures are standardized under one style, I bet there will be a lot more of a cohesive flow with how the world feels in general. We've not been exposed to the whole world looking different due to the changes so we cannot say much about the final product. We may hate individual textures, but have no clue how they blend and feel with the other new textures.
Personally I think everyone shouldn't jump the gun on this and should only really give their final opinion once we get a final product, but my current feel for these changes are skeptical, but very interested for the change. I feel like I will like these new textures a lot, but that may or may not change when I am/we are able to feel out these textures for ourselves in our own worlds.