Hmm... there's quite a lot in this post to unpack.

the current academic consensus on Vth Century Europe is that the Roman Empire didn't decline as much as it did, well, transform.

There is unanimous agreement that the political entity of the Western Roman Empire declined in the 5th century, because it wasn't there at the end of it. However, the eastern half was fine. What did transform was the culture, and although culture changes and develops, to say that it "declined" would be imposing our opinion according to largely arbitrary criteria that are also just our opinions. A monk from the 12th century would take one look the average 21st century music video and think our culture's gone to shit, because there's no religious iconography, half the lyrics repeat rather than being original, and it's probably about sex. Hell, some singers are openly homosexual and work that into their lyrics. To us, this is a liberated culture. To our medieval monk, it would be primitive barbarity.

There also the issue of continuity. If you were living in 5th century Britain, then yes things went to hell. We know that was the opinion at the time because our main source (and frankly, the only one with anything to say) is a sermon called "On the Ruin of Britain". But if you lived in rural Sardinia, then little of substance changed. If you lived in Constantinople, then life would have its ups and downs but was hardly radically worse in 500 AD than it was in 400 AD. Even in the new Germanic kingdoms, there was a lot of continuity. The coins stayed roughly the same, the law codes were based on existing Roman ones, Christianity was the dominant religion etc. That isn't to say there wasn't upheaval, but life moves on eventually and the new people in charge generally didn't want to rock the boat too much.

Thing is, I do not quite understand where that distinction stands, and what makes "Late Antiquity" a useful concept instead of a pretty word to avoid doing a moral judgement on history.

Although part of it is to avoid a modern moral judgement, it's mainly because of this: there was a world outside the Roman Empire, and stuff happened to that too!!!!! What "Late Antiquity" does that "Decline of the Empire" doesn't is put the decline of the WRE in context and allows us to study what came after. It's a time period in which the dominance of Roman culture, language, and power declined in Europe and eventually gave way to new, generally Germanic, kingdoms with their own cultures. A fixation on the Roman Empire is tunnel vision. New Germanic kingdoms rose in the west, and their rise is worthy of study on their own merits, but also because learning more about their rise can tell us more about the empire's fall.

As an example: nowadays, historians say that Roman Art became less realistic and more symbolic due to cultural changes. Why do historians say this instead of determining that artists just got... worse?

Looking at the arch of Constantine in Rome, surely one can see how statues look more rigid than their I and II century counterparts. Even Byzantine Art during the Macedonian renaissance looks quite a bit more awkward than art from the Augustan period... How are we so sure this was the artistic intent, and not just a lack of expertise on the part of the craftsman?

The thing with medieval art is that modern people think it's worse because their standard for "good" is realism. We know that they could do realism if they wanted to, such as the portrait of King Richard II of England or these sculptures, so there's that. But we also know it's deliberate because of the technical proficiency - medieval art looks simple, but it really isn't. For example, how do we know that this is artistic intent, and not just a lack of expertise on the part of the craftsman? I don't think it's good, but I can recognise that there's a high level of technical competency and composition that is absolutely intentional. Art is often trying to achieve something beyond mere depiction, and that is what we as historians are interested in. If you ever get to go to Canterbury, look close at the stained glass windows from the 13th century and tell me they aren't artistic achievements on technical difficulty alone, let alone the overall effect they create. They were extraordinarily talented artists, but they were using religious imagery in a medium that interacts with light to create awe-inspiring effects, rather than doing a portrait.

Another factor is that a lot of the art commonly seen online is from digitised manuscripts, where the drawings are far smaller than they look and therefore more technically difficult than they first appear. It can be tempting to look at a rather crude drawing of some medieval men and look down on it, but when we consider that these drawings are often the size of a post-it note, and usually no bigger than a postcard, then it makes a bit more sense. Manuscript art is also a situation where the artist was concerned with making it good enough so they could move on to their next task, rather than being concerned with making a masterpiece (unless it was a special edition for a duke or something). There's an old answer that goes more into medieval art.

So we tend to call it "Late Antiquity" rather than just "Decline of the Empire" because that encompasses a broader context and acknowledges that our idea of "decline" is not universal and can lead us to make serious misconceptions, like writing off medieval art as crap.