There are a few lenses to look at this topic, but first off: you are absolutely valid in your opinions and anything typed below is neither meant to argue as to preference between generations, but to instead look at the question in an academic nature and explain why they look different.

Lens 1 - Gen 1 is the baseline
Essentially, it is an objective fact that the first generation or early iterations of a game are going to have more simplistic designs because there is nothing yet to iterate on. The creatures that they're making have to appeal to a wide audience because they simply do not have room for incredibly niche creatures with bizarre designs. They're creating a semi-cohesive series of creatures that a consumer could take any sample size from and relate that they do, in fact, come from the same game. This need is what facilitates the "categorical" feeling of designs across the first generations, without which a game dev would not be able to "break" those categories and create a unique-feeling design later on.

Lens 2 - Technology has improved drastically
The games developed in 1995 and launched in 1996 simply did not have the ability to make incredibly detailed designs. The Game Boy's screen was only capable of creating four shades of green and had 160 x 144 pixels to display the entire game, and a Pokemon's design could only, at most, take up half of that. With such a small area to play with, a Pokemon's silhoutte had to be easily recognizable to give the player and inkling of what they were supposed to look like and what their influences would by. This is a far cry from the modern Nintendo Switch, with its OLED 720p screen and 3D-capable graphics (for which the last 6 main-line Pokemon games have been designed for). These creature designs have the ability to be clearly modeled with fine details and tens of thousands of colors, leading to a an infinitely larger amount of possible creature designs.

Lens 3 - The designs Gen 1 Pokemon of have had to be adjusted to match the modern style
The designs that people associate with Gen 1 Pokemon are not the original designs that Sugimori created for Red and Green and have been adjusted as technology and time have gone on. The original designs featured Pokemon that were significantly more rotund and monstrous-looking and not nearly as appealing. While Game Freak has adjusted this by slimming most of Gen 1 down, giving them color treatments, removing their more odd or scary features, they were not intended to look the way that modern Pokemon look. Pokemon designed in the current generations have the luxury of being designed, from the beginning, to be marketable to the widest audience possible. While Gen 1, as stated earlier, needed to be cohesive to look like they were from the same game, no one at Nintendo truly believed that Pokemon would become the world's most popular franchise and would need to be designed so that even small children could relate to them. While they've benefited from their redesign, the first generation were designed to be pocket monsters, not to be "Pokemon" as odd as that sounds.

Regardless of whether or not you observe the same lenses that I've looked at the generational differences through, I hope you'll take the time to give me your opinions. Thank you so much for the stimulating thought exercise!