Yes, but only in the sense that there's a correlation between the need for scaling and the demand for self custody. I believe that a significant number of people want self custody, but I also believe that this number is small enough that the need for scaling is significantly mitigated.

In my opinion, there's no specific size at which a change request should be automatically dismissed. Change requests should be evaluated on merit, not popularity.

Once again, no. Wants are not stable over time nor are they easily predictable. So you can't determine that something is unnecessary simply because the size of the group that currently wants it is small. It's entirely possible that once an idea is implemented, people realize that they actually really like that feature that they didn't think they wanted before.