I was explaining the pros & cons of this on another blog, but my explanation was stretching on so bloody long that I realized it needed to be split off in its own topic. I do hope this will stay coherent, as it's only getting later & later as I try to churn this out:
For starters, let's define a few base rules here, which should allow us to talk about this in general storytelling terms:
1. Bending is functionally indistinguishable from magic. It's not just a punch, it's a punch that shoots fire. Or moves water. Or whatever.
2. A substyle involves Bending something that wasn't previously known to be Bendable, such as metal, lightning, or a human body. It also involves performing a task that wasn't previously known to be possible with that element, such as Healing. The defining feature is that a substyle is a new "spell," if you will.
3. This makes Bending Severing a substyle.
Most of us would agree that Bending Severing was a Deus Ex Machina. So, the first thing I'm going to do is dissect what made it different from the other substyles.
I think that explaining it in this way is clearer than framing it as making the show more or less "realistic," because really, how realistic you find the specifics of mentally controlling the elements is subject to your interpretation of what "realistic" means in this context. Is it the act of inventing a new technique through tedious study that is realistic? Or does the magic aspect make it inherently unrealistic?
We kept hearing that Metalbending was impossible, but here we are. The thing is, Toph didn't actually do anything new, she was simply the first to realize that there was unrefined Earth in metal to be Bended, & be able to locate it. Once she realized this, all she did was basic earthbending. Is it kind of a copout that no one in hundreds of years had done this? Maybe, but the important thing is that the actual power is not out of nowhere.
Basic Point: It only used an old rule in a creative way, so we could accept that it was possible.
Unlike Metalbending, this was a totally different skill from Firebending. Lightning is only superficially similar to fire, it is not formed by, nor does it behave in, the same way. Even so, it was actually foreshadowed from very early on. This means that we didn't have to be informed that it was always possible, we knew from basically the beginning.
Basic Point: We clearly saw what it was early on, so we could accept that it had always existed.
Healing was introduced at the end of Book 1, when we got our formal introduction to Waterbending, as Aang & Katara were only using basic forms up until that point. It makes sense that we & the protagonists were ignorant of Healing, because we didn't know any Master Waterbenders at that point.
Basic Point: There was no one around who should have known about this, so we could accept that it had always existed.
Bloodbending does a bit of a combination. With Plantbending, using sweat for Bending, etc., the existence of Bloodbending was gradually foreshadowed. When it came time to reveal it, it turned out to just be a new application of basic waterbending: The ability to bend the water in a person's body. But it could only be performed during a full moon.
Basic Point: It was gradually foreshadowed, & it had limitations.
Not important or interesting.
So where does Bending Severing go wrong? It doesn't do any of these. It introduces a new rule that no one, not even our resident waterbending expert, was aware of the possibility of, it isn't foreshadowed until the last few episodes ask us to accept several retcons, & it basically has no limit other than "only Amon can do it."
Why is this not a testament to Amon's cleverness? Because we all know that Bryke gave him that skill because Amon needed it to make the plot work. Technically, no one in Avatar invents anything, or has any kind of skills at all, because they don't really exist.
BUT...as authors, Bryke need to maintain the illusion that this is not what they are doing, by providing a form of magic with hard limits & not straying outside of those too far.
Enter the problem that Legend of Korra wasn't planned from the early days of Avatar. It is not possible for them to have hinted at something they were going to do now, so it makes it much harder to sell us on the notion that some new branch of magic has always been possible. What's more, they've introduced so many substyles throughout Last Airbender that the characters are already kinda overpowered.
HOWEVER...if they don't invent anything new, they make it look as though they are unable &/or unwilling to try any new ideas. This is an era of progress, Bending should still be evolving.
This question is intentionally left open-ended. I just wanted to lay out the pros & cons of both writing decisions.
Personally? I hope we get a few things, but if they can't come up with anything that averts the pitfalls that Bending Severing did, I'd rather they stick to what's already been established.