Over time, newer generations will always strive to surpass the older ones. It's also almost impossible to become the strongest. Even after becoming the strongest, there will eventually be a limit. You just won't be able to improve further.
However, whilst you may be stuck, others may be able to catch up and surpass you. Thus, the way to stay the strongest is to annihilate all potential dangers. If no one else is able to improve, you'll naturally stay the strongest.
That's why, every few cycles, everything will be killed and reset. This makes it almost impossible for anything to surpass you.