[friendzis]

> the most valuable resource is rack space

The limit is power capacity and quite often thermal. Newer DCs might be designed with larger thermal envelopes, however rack space is nearly meaningless once you exhaust thermal capacity of the rack/isle.

Performance within thermal envelope is a very important consideration in datacenters. If a new server offers double performance at double power it is a viable upgrade path only for DCs that have that power reserve in the first place.