only a 6.0 on imdb

In movies that's a good score, this isn't like video games. A 5/10 is considered an okay movie, 6/10 means it's considered above average.

Transformers 1 told a relatively simple story

It had three entire separate stories with three entirely separate groups of people, one of which could have been entirely cut without issue.

had clear choreography in its fight scenes

The most popular criticism of the first film is how unclear the choreography is and awful the shots are for it.

a tight cast

It had at least three main characters that could be entirely cut without affecting the core plot. THat's the opposite of tight.

decent enough performances

You're right but this applies to every entry of Transformers, even the ones I think are garbage.

7.0 on imdb

Slightly higher from user ratings isn't really much of a point?

because way more people watch it because it's far better than tranfsormers 2

This is made up in your brain, plenty of movies with 1000 votes are better than movies with 600,000 votes. Number of votes /=/ in any way how good a film is.

reviews and popularity for the series drop as it continues because the movies continually get worse.

Then why did it financially do better every time? If you're going to say "less people voted because they got worse" then I'll say the movies kept making more money because they kept getting better.

My claim isn't true either, but it's the same logic.