1. It is inevitable that someone with much higher skill will come along at some point.
2. does lead 3d artist or any leader should have highest skill level or experience?
3. It is not like you can start switching leading roles when someone better comes along.

1. I don't think it is inevitable.
2. No. The lead artist (or any leader) should have both high skill/experience and leadership ability. Skill/experience alone does not make a good leader.
3. Why not? Is this an amateur operation that wants to stay amateur, or is this a professional endeavor, a business endeavor?

people in the team are not paid

Ah. I see. Well, good luck!

I hope your team has a collaboration agreement in place (see this forum's FAQs).

I’ve been on a number of similarly sized amateur teams and the way I’ve usually done it is that the lead artist or programmer is primarily the one who sets the artistic or mechanical direction of the game and delegates specific tasks to other members of the team. Usually the most experienced and most skilled person is the lead, but this isn’t always the case. (That being said, unless someone clearly has much better leadership and management skills, it’s probably best to start out with the most skilled/experienced programmer or artist as the leader of their respective group.)

Even in a highly collaborative team, everyone is going to have a slightly different idea of how to do things, so the main responsibility of a leader is to decide on a single course and keep everyone else on track. The lead artist should have the greatest understanding of the overall art style of the game so he can make sure the other artists’ work fits together in the game, and the lead programmer should have the greatest understanding of the program’s architecture so he can coordinate other programmers working on smaller parts. This is also why it’s probably not a good idea to switch leaders in the middle of a project, except for extreme cases. If the new leader has a different idea for the art style or how the program should be structured, it can cause conflicts with all your previous work and really mess things up.