Resizing levels vertically downwards makes perfect sense, and I actually do it quite often... As long as you can't choose which edge to add/remove length from, sometimes you'll have to move the level. For that reason, I don't think simply changing which side the change is made from would really fix anything.
I see your point, but my experience of it is that I generally always want to raise the top edge rather than lowering the bottom.
Also, what you've said doesn't address the issue of wanting to shrink a level: if you do so without moving the terrain pieces first, they disappear off the bottom edge of the level. This absolutely wouldn't ever happen if vertical resize was top-edge dependent, even for those who design top-down; it's way more likely that, when shrinking from the top, it's either being done to remove empty pixels or to define the top edge having placed all necessary terrain and objects.
It absolutely wouldn't ever happen? What if the level has a ceiling, such as this one?
This level's vertical size is mostly simply due to the fact that the level provides a lot of stackers, and I wanted to ensure that they could not be used to make the fall into water safe (well, if you were to platform over the water, anyway). If I wanted to re-shrink it, however (suppose namida adds the
downdraft object I suggested earlier and I wanted to change the level to use it, for instance - this was the level I was working on when I thought of the idea, after all, though at this point I'm used to its size and I'm not sure I'd change it back to its original size, but I digress), then parts of the level would go offscreen if I don't change the terrain first. Note that I haven't specified whether the change happens from the top or bottom: in this case, it doesn't matter - both options would cause parts of the terrain to go offscreen. This is true of any level that has a ceiling. It's not unusual for levels to have open-air ceilings, in which case the top boundary doesn't really have anything going on, but it's also not unusual for levels to have death pits of some sort at the bottom, either - in which case you might want to trim off the excess space at the bottom.
One of my more common use cases for extending a level is extending fall distances, whether to make room for a fatal fall, or for increasing the length of a fall such to make it harder to backroute the level with builders/stackers, if I didn't intend for the player to use them to break the fall. As such, the top-anchored components should maintain their current position relative to the top of the level. Of course, so should the bottom components in many cases. From a convenience standpoint, a simple way to choose the anchor point for the resizing is probably best: this way, we can choose whatever will cause us to have to move the fewest components. Of course, if you like having solid walls around all the boundaries like I do in a lot of my levels, then you're pretty much going to have to move some level components no matter what.
Suppose we want to make this level bigger, but keep the overall layout similar: no matter which edge you move to extend this level, something will have to be moved.