Some level of uniformity is fine and indeed the norm. It was uniform back when posters were hand painted, eg 19th century world fairs or zoo openings or theater posters. They look pretty similar. Carneval posters looked similar.
The expectation of some kaleidoscope of styles is not really grounded in how things used to be ever.
Also, it's just nostalgia, just like how some young people now think audio cassettes and discmans are "cool" and have some revival, some are nostalgic for the back-in-the-day Word posters because "at least back then", but "these days" it's decline etc. Pretty sure in a few years people will be nostalgic for crappy AI slop of 2025-26 because their shittiness will feel like some charm compared to whatever the AI will be able to do.
Sorry, but Walkmans and Discmans were considered cool... At the time.
Personally I like simple designs. I always remind people who make posters that whoever looks at them will only do so for a matter of seconds. If you want extensive detail, put it in smaller print. Put the main hook in large letters and a brief description.
But sometimes I also purposefully made myself some Comic Sans stuff, exaggerating them in every possible way, as a joke. And it was well received because people knew it was my own work and understood the joke.
If I made this today, it would not be seen the same. People would thought I'm trying to fight gen AI trash or I'm nostalgic of the past. While I'm just trying to make a joke by making something awful on purpose.
And I couldn't do the same joke using gen AI. The "on purpose" cannot be conveyed. People would just see gen AI trash.