You are right to wonder about this and doubt the relevance of such a statement. You are right that the entire legal profession must be considered.
You talked about a lawyer with ten years' experience earning $50,000 or $75,000. She may be excellent and may be doing remarkable work. After ten years in practice, she may even be a candidate for a position on the bench. So, why would we automatically rely on the idea that in the past, a great many judges have come from large law firms? That is not a good reason.
Increasingly, there are large numbers of public servants working in the legal branch of various departments who are doing excellent work. They are not deputy ministers. They act as counsel, general counsel, and so on. There are also lawyers working in legal aid clinics.
In the field of family law, I know of some judges who came from legal aid offices and are excellent. They may have a greater appreciation than private practice lawyers for family or social issues facing litigants.
That is the kind of person we should be looking for. So, you're right: we need to consider a much broader range than just those who have supposedly had successful careers in large law firms in Montreal, Toronto or Vancouver.