I think your point is right for 111 8th or Equinix/SV1/Great Oaks. But for other sites with high value peering, including EQIX in Ashburn, Coresite, your limit

I think your point is right for 111 8th or Equinix/SV1/Great Oaks. But for other sites with high value peering, including EQIX in Ashburn, Coresite, your limiting rate is likely to be the power, not the space or power. I.e. they'll "give" you 1 cabinet for every 15kw you buy. So my assertion assumes you're doing large number of dense cabinets. If one is doing a large number of dense cabinets, they almost certainly should not do it at a high value peering point, and should backhaul it. You should be able to get diverse metro dark fiber for <$5k/mo if not substantially less. Put a single cabinet (or pair for redundancy) of $2k/mo cabinets in Equinix or Fisher and off you go.