We already do this. We actively introduce new founders to "alumni" that can help them. There are now a lot of them. At the first dinner, instead of having an outside expert speak, we invite all the alumni. This time there were more speakers than audience at that dinner.
The peer-to-peer aspect of YC already scales. The hard part to scale is what we partners do.
Considering that what the partners do is the most valuable aspect of YC, I don't see this scaling without adding more partners. This begs the question of what YC is: is it a company that forms more companies, or is it an angel-firm that creates more angel-firms?
The latter can work only by forming these new angel-firms in different startup-beds. There are only a limited number of startup-beds, and a limited number of connections that can be used by partners. When these resources are used up, the only choice is to move to another area and start again.
The former is what YC already does, within the limits of the partners. Adding new partners could increase the number of companies funded each year, but only to a limited extent. Finding a partner is hard because it needs to be someone that won't disturb the current dynamic and who will bring in new connections/resources that aren't currently represented.
Frankly, I don't see any way to truly scale what YC does. Encouraging startups to form is limited by its very nature to within bounds formed by the market. These bounds have not yet been reached, but still exist.
The peer-to-peer aspect of YC already scales. The hard part to scale is what we partners do.