There are multiple ideas and we're all still actively evaluating them, with page after page of ongoing analysis. Why so quick to draw conclusions?
The existence of an effective counter action doesn't mean an attack presents no issue. Consider rock-paper-scissors: every attack has a counter, but the attacks are still an issue for the defense, because no single defense can counter every attack. It's my hope that 7DtD will eventually have enough kinds of attacks that there can be threats to the player in all scenarios. E.g., your stilt base is safe from diggers but not from fliers, your underground base is safe from fliers but not from diggers, etc.
This is not the issue! Besides which, it's wrong. The developers set the rules of their universe. Of course they can come up with a rule set that has no defense. They can add lightning strikes that kill you no matter what. But they won't, because this is not the goal.
Actually it's way, way less magical than all that terrain being stuffed in your backpack. The latter brand of magic is so legendary, we have a name for it: hammerspace.
You wouldn't see the digging zombie appear. It would just skip over the boring and performance-hurting part of slowly chewing through many blocks of terrain, and get to the interesting and detectable part where it's about to break in. When done correctly, you won't be able to tell the difference.