Whether it's a flashback or not is irrelevant. The aspect of length is fairly subjective, but in terms of why the Golden Age may be seen as more enjoyable is possibly because of the pacing.
It's not irrelevant since it's the reason for the pacing. And no, the "aspect of length" isn't subjective. It's objectively defined in terms of volumes and episodes (i.e. roughly 10 volumes, as you said yourself). The pacing however could be said to be more of a subjective thing, a matter of perception. And indeed, I could make a case that the pacing of the Golden Age arc, aside for the time skips (which are directly related to the fact it's a flashback), isn't too different from that of the rest of the series.
Foolish to you, but not to those who think the Golden Age is the best. From their perspective, they like the Golden Age more just by the fact that Guts wasn't branded till the ending of that arc. By being branded, it demanded for the story to have demons hunt Guts down for sacrifice. They may not understand what Berserk is truly about, but even if they do understand, they can still like one aspect of the story (where Guts is not yet branded) more than another aspect (Guts being branded).
It is foolish, period. Berserk is a fantasy story about a man who fights monsters. That is made clear right away, from page one of volume one. To pretend the story went "downhill" after the Golden Age arc because of the reason you invoke (that there are "too many demons") requires one to also pretend that the Black Swordsman arc didn't exist. And generally-speaking, that usually includes omitting some supernatural parts of the Golden Age arc as well (like Wyald's segment). It is an exercise in self-convincing where people try to make Berserk into something it never was, and that certainly is very foolish of them.
I suspect a lot of people who started on the Golden Age Arc (particularly some of the people who started with the anime, or started the manga or anime after the Golden Age arc flashback had started) felt like the Eclipse was a bait-and-switch. I've heard people who only watched the show say the Eclipse "felt like it came out of nowhere". So "all of a sudden" this cool show about this band of medieval style mercenaries they were digging becomes a supernatural story.
You say
particularly the people who were introduced to Berserk with the TV series, but the truth is, it
only makes sense for those people. You don't start reading a book series from the middle. That being said, even though the supernatural element was toned down in the TV series, it is still present in it, and that includes the very first episode, which features Guts as the Black Swordsman.
One could certainly argue that this is the wrong way to approach Berserk because it's not how the story was intended to flow, we meet Femto before Guts' flashback after all, but again there's no accounting for taste. The mostly mortal military stuff was what attracted some section of people to Berserk.
This is more a matter of criticizing how the team in charge of the TV series decided to adapt the manga's story. However, regarding the way the story's meant to flow, I don't see how taste plays any role beyond someone hypothetically liking the TV series but not the manga. The story was created a certain way, and outside of the differences induced by an adaptation that can't be changed.
As a thought experiment, I could see how someone who was introduced to Berserk by the TV series (released back in 1997,
17 years ago) might be disappointed by the fact the manga features more supernatural elements than the animated series. And that's cutting them some slack for forgetting about the first episode, about Zodd, about the beherit... And then disliking the Eclipse. This imaginary person certainly is very picky even at this point, and clearly only partially enjoyed the show. So anyway, maybe that person will then not like the manga, and that's that. They prefer the bastardized animated version. But then if they read the manga, well things are different. Even the Golden Age arc is a different experience, since many elements were omitted from the TV show. And at this point, the reader is expected to have read the Black Swordsman arc and to know what the series is about. Plus they already know what to expect with the Eclipse. Therefore there remains no justification for feeling that "it came out of nowhere".
And some people are probably pissed their favorite characters died and were later "replaced" with ones they didn't like so much.
That's a pretty stupid way to view things, and in order to even start being able to form that opinion you'd have to stick with the story for over 10 volumes after the Golden Age arc ended. It just doesn't make sense.
