Context is absolutely key for me here. The first thing is that being too much outside your comfort zone is damaging: as you say, it focuses the mind but that requires extra energy. Making sure people are comfortable gives them brainspace to move out of their comfort zone in a structured way — which leads to my second point. If they feel supported, they are generally more willing to take that step into (for them) the Great Unknown.
All of this requires knowing your people extremely well, and drilling into the root of why they don’t want to step outside their comfort zone now if that’s the wish they express.