If you really want to get into the conspiracy theories, you could say that they are changing the textures to try to get the legacy edition to slowly fade away so people only know of the new version where you have to pay for texture packs, skins, shaders, etc. Oh wait, they're already trying to make the legacy editions fade away.
So, because a few of the block textures look bad we need to change almost all of the item, block, and mob textures?
Legacy Edition? what Legacy Edition? Oh is it the Java Edition? well is not fading away anytime soon and I don't think I need to do some explanation of why(also because I simply just suck at explaining things without bad english stuff).
Frankly, over the years the style was always maintained pretty well. I find it hard to see how they "look bad" in any real sort of way. he textures of Minecraft helped make it what it is. You can show a texture from Minecraft to someone, and bang, they'll be able to tell you on the spot which game it came from. These new textures though? Not only to they look appalling, but it just tramples right over the legacy and style that Minecraft possessed. My gut is wondering if that's what they're out to do.
Potential tinfoil aside, I'm still blown away that they didn't hire a better artist, especially with the money they're sitting on and supported by.
Some of them looked to detail and some are straight up bad(looking at you andesite, diorite, granite, and the diamond block). Personally tho, I liked what I'm seeing so far, the new textures looks so much better(to me anyways) and much more consistent.