What would have happened if Dende could have made new Dragon Balls?
It would take longer, but they would appear eventually. Humans would keep on wishing for stuff, adding to the negative energy stored in the balls. So I guess they could renew the balls from time to time, preventing the negative energy from building up, but where would the energy go and what would happen to the balls? The negative energy is still around, so I think even if the balls would be renewed over and over again, the cumulative negative energy would eventually make the seven dragons appear. I think it was Earth's destiny for it to happen. We should all be happy that at that exact time, Goku was coincidentally strong enough to defeat them. Imagine what would have happened if he was still only a boy.