I can't speak for every edge case, but you already basically ruined your argument when you mentioned "rebuild the missing half". That involves work, and your asking for a zero work, maintenance free, zero risk underground scenario, and that isn't happening because it would take a bunch of work and be bad for 99.9% of players who bought a survival horde crafting game.
I beg your pardon?
First, I was so safe that I was able to undertake reconstructing the whole building. That came after the fact.
Second, have I not recently stated the amount of hard work that goes into underground building, especially for those going above bedrock? The stamina requirements and associated food, the experience requirements and perk purchases to get through the rock at a half decent speed, the huge land shifting that goes into making a space in which to build, the need to return to the surface for crafting materials to actually construct stuff like the forge, the workbench, chem station etc - the list goes on...
Third, a bunch of work? You mean disabling zombie digging?
Fourth, I don't know where you get your stats from to claim 99.9% of players who bought the game or how it would be 'bad' for them. And what do you mean by 'bad'?
Why don't you do a poll on steam or somewhere you can gain genuine and large sampled votes on things players would like?
But even if it were the case that even 99.99% of the playerbase didn't build underground, then it wouldn't make a difference having the option to disable zombie digging.
If people aren't building underground or they didn't buy the game 'to be safe', then it begs the question of why you would bother to put such digging measures in place at all.
And fifth, and finally, I most certainly am not asking or expecting anything maintenance free. I enjoyed making underground bases to avoid hordes that I didn't want to fight as much as I did maintaining an above ground base to take them down when I wanted to take them on. That included upkeep, traps, and replacing damaged ground.