I have to say that personally, I'm not a huge fan of that animation
as part of orig_fire (this last part is important - I do like the graphic itself, I just don't think it fits into orig_fire as a replacement for the firepit very well). I also would agree that this graphic solves the issue for the left and right sides, but if I were to encounter that graphic sitting on top of terrain, I would at no point even consider the possibility that lemmings can walk past underneath it - if anything, I think this new graphic is
worse at communicating that. Thus, I don't feel that that animation in any way solves the issue at hand here (however, I do feel it's a good animation for use in a new, custom object, with an accurate trigger area).
Any graphic proposed needs to (a) very clearly fit the existing trigger area (if changing that was still on the table, then solution: change it and keep the animation exactly as-is), and (b) fit into the orig_fire style in terms of color selection etc. Beyond what's needed to achieve those, it should change as little as possible.
As the objects were unreachable this is anything but not a fix. The change did not affect the level gameplay (no fixing) and broke the visual aspect.
This level previously had a fire object that looked like it should be deadly, but could actually be walked under. It now has a fire object that's just as oddly placed as it was before (namely, placed such that if a lemming were to reach it, the lemming could safely walk past it), but now has visuals that reflect that. In this particular level, it's trivial to see that one fire object is unreachable and the other two aren't important to have accurate triggers (because if the lemmings miss the fire trigger, they fall out of the level anyway instead).
Let's look at some of the others. 0419, for example, every fire pit
is reachable. 0414, which you mentioned as an example that breaks, actually does the exact opposite - the fire pit now sits perfectly on the ground instead of being slightly buried in it. 0319, the fire pit is definitely reachable and at least at a quick glance I would not immediately rule out the possibility of going near it; this change has made it clear that should you reach the area it's in, you can safely walk past it. 0315, most of the fire pits are accessible and clearly intended to be obstacles - and again, these don't really break as such, though one does look slightly awkward.
I didn't look at every example level you mentioned, and there are some where I do agree that ultimately it didn't provide any benefit - such as 0404 - but I think you are very much overestimating the caused problems vs solved them ratio of this change. Ultimately, I still feel just as much as I did before that, at worst, this change will trade a physics clarity problem for a visual oddity one, which is less severe by far.