Then I build the next base design and the next until I run out of ideas.
I'm not looking for an adrenaline rush while playing. Excitement, tension or emotional roller coaster rides are not my goal.
In A15 I dug once passable tunnels into all biomes of the Navezgane map. Just for fun. And I have removed a mountain top to build a big mountain fortress on it. You'd probably die of boredom, but it's great fun for me.
So don't worry that I have no reason to continue playing after I have a base that reliably fends off zombies. That's when the game starts to get interesting for me.
Not if you have the base well planned. Of course, if you call a wooden box surrunded by spikes a base, I'm not surprised if the base is overrun.
I've invested a lot of time in my current base, testing different designs against hordes in creative mode that were much stronger than what I usually get in the game.
In my opinion, a game is not defined by risks. There are games where there are no risks and no challenges but they are still fun.
Taking risks is in my opinion something for people who can't plan properly or who are bored. Methodical planning can eliminate many risks in advance and if you have considered everything even all risks.
A puzzle is also a type of game. A game that requires patience and logical thinking.