Roughly when did it become fashionable to trash this movie?

I have not come across any prequel fans trashing A New Hope. However, I have read dozens of threads in which fans of "The Force Awakens" trash it. Usually it is when someone points out Rey's "Mary Sueness". Then everybody will start saying that Luke was also a Gary Stu, which isn't true even in the least bit.

What is a "good" action film? That has changed over time imo.
In the 1970s an action movie was in the tradition of 5 to 10 minute final climaxes of action being set up by long stretches of story, character development and dialogue.
Old war movies, samurai films, westerns, sword and sandal films, all had this formula.
- In the 70s the modern 30 minutes+ of action sequence did not exist yet.

- Star Wars A New Hope follows this old formula. (As does the rest of the original trilogy.) For its time ANH was an action movie; With relatively brief moments of action; getting Leia out of prison, the trash compactor, the dogfight leaving the Death Star and the final trench sequence and the destruction of the Death Star.

- The modern action movies I think began with James Cameron.
In "Aliens" from the middle on there are long stretches of action stitched together with only brief dialogue scenes.
This has been compared to a roller coaster ride in some discussions.
Several directors copied Cameron after this and Emmerich and Bay did it with little character development. 
Less thinking by viewers makes the action more intense like a video game. 

In the Star Wars prequels probably Attack of the Clones was Lucas' way to try to imitate this. AOTC from middle to end has long stretches of action with little story. Rogue One has that with disposable characters. 

* Today the Star Wars fanbase is split. 
- Some want character focused SW like the original trilogy and TFA. (Lawrence Kasdan wrote Empire Strikes Back and TFA.)
- Prequel and Rogue One fans can find the old SW style boring. They don't care about the characters as much and want plenty of action.

Imo at least, BB ;-)

I was 13 in 1977 when this came out.....have never ever had anything remotely approach being as blown away as watching this in the theater in May 1977. This movie shall not be trashed.

Yes, it is that too. Kids just want an action montage. When I was young and watched war films or action films, it wasn't enough for me just to see the fighting scenes. I wanted to know why they were fighting, what led to not just the overall conflict but what led to that moment itself.

When I watched the pre-quels I was just underwhelmed by the amount of fighters they crammed onto the screen, it looked like a mess. There is one scene where the Wookies I think are storming some stronghold and it's so crowded. The light sabre fight with Yoda flying about is a joke as well.

Not just a lack of attention span but a lack of emotional depth on the part of the viewer as well.

I agree that the death of Han was not handled as well as the death of Obi-Wan in ANH. 
But at least with The Force Awakens I felt a similarity with the original trilogy. It's not as good as the OT but at least it's in the ballpark, with Chewie's rage after Han's death for instance. 

Compare that to Rogue One where multiple main characters are just blown away (literally) without any comment. After a death in RO, there's just more action until the next death and then more action again. 
- I sense a big difference between that video game feel in RO compared with TFA where the tears and sadness by Rey and the pain in Luke's face tells me that a horrible thing has happened and that the loss of life meant something emotionally to them.

Imo at least, BB ;-) 

PS. And the CGI Tarkin and Leia in RO? Definitely looked like video game characters.  

One of the Greatest Achievements of 20th century Cinema.

Editing- superb,
Dialogue- superb,
Cinematography -superb,
Acting- superb,
Groundbreaking effects -superb,
Characters -superb, 
Set designs -superb.