Now THIS is the thread where I wanted to ask what you all thought.
'Tis funny; one of my neighbors asked me about this over the weekend, too.
It doesn't hurt Lovelock's public profile any to make crazy predictions that can't be verified -- if human population does drop to 500 million, by the time it happens I doubt anyone will be able to determine that Lovelock predicted it, and the situation would be so dire that none of the survivors would care.
Having said that -- I liken the relationship between Earth and humanity to the same relationship between my family and my house. There are a lot of systems in the house that we take for granted, which make the "livability" of the house comfortable, allowing a routine performance of family activities within the house. If one of those systems becomes significantly impaired, livability becomes more difficult and requires a greater expenditure of energy to compensate.
Imagine if the water system became contaminated with a dangerous toxin, with sufficient toxicity that exposure to the water had to be dramatically minimized. Drinking water would have to be imported; cleaning (clothing, personal, housewares) would have to be done elsewhere; waste removal would have to be done elsewhere; preparation of meals would be affected; etc. We would probably conclude that living in the house under such conditions was unacceptably difficult, so we'd find other, livable conditions until the problem was fixed (f it could be fixed).
So what if we couldn't move? What if the house was literally the only place we could live? Then we'd have no choice but to come up with some way to fix the house environment -- i.e., remove the contaminant from the water.
One of my points should be obvious from the previous paragraph. Speaking more to address Lovelock, humanity's presence on the Earth is tied to a number of important ecosystem components. If some of these components become impaired, then the Earth's livability will be affected. If one of the critical systems gets really impaired, then a lot of people will be affected and significant human suffering could ensue.
As a final "gotcha" point, think how dependent we are on insect pollination.