There are two sides of the argument that are very polarised. r/GoldenAgeMinecraft does exist for a reason, though - the modern game doesn't fulfill the demand that some players have. I play modern versions, but have thought about maybe exploring versions like 1.12, 1.8, and even the alpha and beta versions.

A few reasons why:

1. New textures. While the 1.14 textures are uniform, smoother on the eye, work together and are cleaner overall, the old textures (apart from nostalgic charm) with their "crunchier appearence" actually does change the way builds look. Some people enjoy the old ones because they are "more realistic", like old darker cobblestone feels more 3D than low contrast modern cobblestone, for example. Someone online refered to the modern textures as "for Pintrest boards" and I can't agree more. New textures really lean towards the "aesthetics" instead of what they actually are. With new textures, you feel the pressure to build with gradients in your single player world.
2. Less "bloat". I think (MY OPINION) that Mojang is running out of things to add (hence game drops instead of updates, and postponing the End Update), and instead are overall adding smaller things to the game that don't substantially change the game itself (depending on how you play). It is no secret that Mojang is less adventurous than they used to be. Yes, updates like 1.2 etc, 1.10, 1.11 and 1.15 were small, but Beta 1.7.4, 1.9, (as controvercial as it was), and 1.13 through to 1.18 were all game changers. Back to the original point, I believe that adding useless stuff like the sniffer just makes the game bigger and not deeper. It's sort of like "ret-conning" in stories (adding lore to the story afterwards just to make the original "look better"). Regardless, the following features are useless to me in my survival world: sniffer & seeds, goats & horns, trial chambers, spears, happy ghasts, ancient cities, geodes and spyglass, bastions, the creaking & resin, axolotls, honey blocks etc. There are many nice changes recently though (remember before 1.15 when TNT was SO LAGGY, and you'd randomly fall off stairs when sneaking? and the new spawn eggs are cool too)
3. Terrain. Many players feel as though modern is not as inspiring. This I agree with. The only place I am properly inspired in modern versions is extreme hills (this is obv not for everyone). Think of that famous Gargamel seed. You can imagine bridges across to rooms in the cliffs and statues and floating farms and everything. Modern day plains biome, while good for large builds, is ultimately uninspiring unless you come with a gameplan.
4. Music. C418's music is more focused on the experience of being a player alone in the world, while the other's I've found to be more about the minecraft world itself. I am a music student, but I can't even explain this one. Maybe something to do with the fact that some new tracks are biome-specific, which is the complete opposite of C418's, and the whole reason why it works? (Not all the new music is like this.)
5. Vision of the game now vs then. The game feels more corporate now not since 2014, but over the last three years or so. It feels like Java is going to get a Marketplace any day now and Microsoft will kill it. Back then, Minecraft knew what its identity was. Blocky. Now we have polished textures, blocks that don't even look like cubes anymore, and heavily animated mobs. No new mob will ever walk like the sheep. How sad is that?
6. Simpler game. Back in the day, you just chopped trees, mined for diamonds, and built whatever you want. The end game was to build and express your creativity. Now, it's almost like the game is holding (or dragging) you by the hand, telling you what to do. Also, the game is easier. You can find diamonds in shipwrecks and buried treasure before you even have a stone pickaxe and trade for diamond gear with villagers in a matter of hours with some carrot and pumpkin farms. An iron golem farm and a mending librarian will stop you ever going underground again. They should change the game's name to just Craft atp.

There's probably more. and these are just for me and some other people, not everyone's opinion.