No, the sequels in trilogies that have a highly successful original release don't usually make less money. LOTR isn't the only exception. The only other trilogy in my lifetime on the same level as Star Wars besides LOTR was The Matrix, and the 2nd movie made way more than the first because the first was so good. The third shit came up short because the 2nd film shit the bed.
Forgot about Matrix Reloaded, perhaps because I wanted to forget about it.
It usually has more to do with how the previous movie was received and the amount of hype & marketing.
Also, no, Episode 3 of Star Wars made more money than Episode 2 because people wanted to see Anakin become Vader. It could have gotten miserable reviews, and the people would have still gone to see that. That was the entire point of the prequels. Besides, you're only further strengthening my point. Despite Ep 3 being received more favorably then Ep 1, not just Ep 2, it made less money than Ep 1. That's because people hated it. SW's reputation at the time was at an all-time low with the mainstream following the prequel trilogy.
Comparing a SW movie that only came out 3 years before the previous SW movie, to the SW movie that came out 16 years after the previous SW is a bit unfair.
Ep1 was a 'culture event,' in which practically EVERYONE that wasn't a SW fan or general movie-goer went to go see it out of curiosity... or to see beautiful eye-candy.
Similar to how Avatar was the first modern CGI 3D movie.
By 2005, big-budget movies loaded with CGI were commonplace after Ep1 set the Hollywood standard of 'everything must be CGI to attract audiences.' Even Anakin turning into Vader, in addition to the expected CGI-fest, couldn't pull in giant audiences.
'Everyone hated it' is generalizing a bit. Rotten Tomatoes has a 66% audience score and a surprisingly high 80% critics score for Ep3.
https://www.rottentomatoes.com/m/star_wars_episode_iii_revenge_of_the_sith
Where did people rant about this? In real life. Newspapers, too, but anyone who talked about Star Wars that wasn't a Star Wars geek talked about how bad it was. Even when I talked to other SW geeks most would shit on it.
Well yeah, because with Star Wars there's a learning curve to understand it.
For example, most non-fans don't get that the characters AREN'T SPEAKING ENGLISH (or any Earth-language) to eachother. It takes place in another galaxy millions of years ago, and they're speaking 'basic.' Not the absolute best excuse for what most would consider imperfect dialogue, but for a space opera it is a decent excuse for most scenes.
As for the SW geeks disliking it, I'm not surprised because there were several excellent SW books released during the 90s that no possible prequel movie could have been satisfactory to when comparing them.
And after reading EU books, that's the standard of quality in the fan's mind who read them. I'll go further into that tomorrow.