What's wrong with less people on the planet?
From which angle though? Functionally, our biggest problem with our level of population is energy (energy = food and water, as long as you have access to sufficient energy, the other two are easier than you'd think to produce on massive scales). Yeah, if there is 7.5 Billion people living like a North American does with the current technological state of energy generation, then we have a big, big ****ing problem. Thankfully though, the developing world seems to be skipping that part of our development and going straight to renewables, which should significantly lessen future burden on the planet as their quality of life increases.
The next aspect of this discussion is connected to that....as quality of life increases, birth rate decreases. Tons and tons of data available on this. You want to see fewer people on the planet? Support women's education initiatives and the low hanging fruit on the disease tree (bed nets in malaria stricken regions, etc). Women's education is the single biggest indicator in future child births (smarter, more educated women want to have fewer children regardless of which region or religion they're in and enough of them fight for self determination that they tend to win it for the whole).
The elephant in the room though is simple age demographics. Short of some sort of incredible advance in medical science, large swaths of the population are set to start dying off over the next 50 years, stabilizing population growth rates overall, with most projections putting us somewhere in the 8.5 Billion range in the back half of this century. Even that, imo, doesn't account for the incredible potential of women's education and overall quality of life improvements in 3rd world regions.
Long story short though? As long as certain technological/social/economic trends continue, "over population" is a lark.