I would say Alan Watt's reasoning is fundamentally still sound. He ended up being wrong when he predicted nuclear or biological warfare. With our current understanding of technology, land use, and consumption habits, we know we can easily consume less and put less strain on our environment. Overpopulation is only a concern if you consider our current consumption habits to be necessary, but I don't, so I'd say he wrong there as well.

He was spot on when he said "pollution of the atmosphere, contamination of the water and the erosion of our natural resources" Any group that neglects to live in harmony with these things is bound to end one way or another, sooner rather than later. Though I don't think even he grasped the full scale of the problem.