The mid-1970s are now almost 2 generations past, and every component has been absorbed into the culture and re-emerged many times. Like Star Trek a decade earlier, all those things that were exciting and spectacular when they were new now look painfully trite and cliched to 21st century eyes.
Star Trek managed to renew itself with first the movies in the 1980's and then TNG/Voyager/DS9 during the 1990's. I think that Star Wars had, several times.
Most (who have been) Star Wars fans were not even born in 1977! Myself, I grew up with the original trilogy on VHS and when my fandom got reignited in 2002, I met a lot of fans who were 5+ years younger than me — and not all of them were fans of the original trilogy first, like me.
I have also seen that the Clone Wars
did attract a new wave of young fans, while also being well-liked by previous generations.
Many [weasel word][citation needed] didn't find the prequels held up to even the originals and that was 15-20 years ago so I'm not surprised there isn't the same cultural hype over the brand there once was, though they're still very decent money makers.
Yeah... and some people did not even think that Return of the Jedi (1983) held up to the previous two films, because the Ewoks were too cute and Jabba's palace had too many muppets. But each new movie has kept the majority of the old fans and brought in new.
The sequel trilogy has not brought in any significant number of new fans, while alienating the majority of the previous generations of fans. There's a big difference.
I feel like Lucas realized he couldn't keep up with trying to keep the entire universe cohesive anymore
Lucas never did. Other people at Lucasfilm did. Lucas did his own thing ("canon"), which was the movies, and another group at Lucasfilm created the (non-canon) "Expanded Universe" in books and other media by filling in the blanks from what Lucasfilm had told them.
In-between the prequel movies, the EU group did an amazing job producing an expanded story coherent across multiple media, in sync with Lucas's main story from the movie scripts.
The Clone Wars animated series was again Lucas' own thing — which is why it wasn't entirely coherent with the expanded universe.
The main writer and director on the show, however: Dave Filoni, was a new guy but a long-time super-fan who knew the Star Wars lore including the Expanded Universe inside and out.
He is also one of the producers of the Mandalorian, so there is some hope... albeit slight.
The EU should never have been wiped.
No, and it wasn't supposed to have been. When Lucas had left, the plan was first to unite the "non-canon" EU into the canon and make it into a coherent narrative. I attended the EU panel at Lucasfilm's own convention in 2013, and saw and heard the people in charge announce this!