I don't put as much stock in Musk as many do, though not suggesting for a second he isn't a very intelligent man. As well, I tend to agree with many of the things I've heard from Peterson. That said, I do not agree with the idea of population collapse being a threat. Interesting that Musk sees it this way, considering some of the innovations he's involved in, as they would seem to have the potential to facilitate the very thing he earmarks as a threat.
As for Gates, I wasn't necessarily aware of his position on the subject. If that is the case, I'm quite sure it's the only thing we would agree on, I see him has one of the most potentially evil individuals alive today and have never been fan of his. Ever.
The bottom line is, the total number of people is not decreasing, it only continues to increase. We know this. It would almost seem like the 10-11 billion calculated max was/is seen as some kind of target, but it was intended as a warning. We have been seeing the effects of hurtling headlong towards this "critical mass" for quite some time now. The more people there are, the more the effects of how we conduct ourselves is amplified, and I feel that can be applied to almost any aspect of human existence on this planet.
Now, if we consider the demographics on who is and isn't reproducing, well, that is a much different (and more sensitive) matter for debate. It would seem there are socioeconomic factors at play here, where some groups are being more thoughtful about reproducing in numbers (if at all) and others have no consideration for the matter...