>>723652568 (OP)
If you leave a gap in there, there'll be a cave-in eventually, you can't have that. (I know that Minecraft does not allow this to happen with non-falling blocks (even sand blocks will stay up as long as they remain unperturbed), but I can't stop imagining myself or God forbid some innocent bystander walking past that spot and falling down because there isn't anything supporting the ground from below.)
>>723652704
There's a simple solution to this: you can take materials from around the hole as replacements for the lost materials. Just make sure to leave it in a natural looking state. There might be some dirt blocks with no grass left once you're done, but they'll get covered in grass over time, so you don't have to worry about that.
>>723653018
In Minecraft, monsters tend to appear in dark places. If you leave a big enough gap underground, the pitch black area will be a perfect breeding ground for monsters regardless of the time of day. Torches light up places and keep monsters from spawning in the vicinity.
