Imagine this: you think you're doing great in the world, and all your friends are telling you you're doing great in the world. But one day, you look outside the bubble you're in and realise just how far behind you really are.
This is what complacency leads to. By limiting your own perspective and telling yourself that you're doing great, you don't notice how everyone else is capable of so much more than you are. Even if you once had a near-insurmountable lead, by no longer applying pressure to keep moving forward, or by reducing pressure to the point that you feel like you're moving forward but are practically staying still, anyone can catch up should they decide to.
It is important to avoid being complacent. Never think what you're doing is completely enough. Continuously push your boundaries. Like in Maoism, "constant revolution" is needed to "prevent counter-revolution". Constant progression is required to prevent stagnation.