Actually, many people consider it. Some are simply cautious about hosting their data with a third party. Some are prevented from using a third party for compliance or regulatory reasons. Also, it's generally more cost-effective for extremely large datasets to be self-hosted rather than hosted by a third party.
Aren't the bulk of customers who turn to the cloud rather small operations, who are trying to "outsource" as much of their infrastructure issues as possible? And aren't these customers much more concerned about pricing, rather than possible future growth?
Note: I don't mean to ask this sarcastically. I'm actually asking.
From my experience working with Rackspace Cloud Files, customer sizes are all over the map. Some customers are very small. Some are very large. I know that S3 has a similar variance in customer size.
From my experience talking to users (and potential users) of Openstack (http://openstack.org), there again is variance. Most people are relatively small (a few hundred GB to a few hundred TB). Some are much bigger (several PB). The most exciting thing I heard was that CERN is evaluating Openstack swift (http://swift.openstack.org) for their storage needs. A researcher from CERN gave a keynote at the last Openstack design summit. CERN generates 25 PB / year and has a 20 year retention policy. They have vast storage needs. The storage needs vary greatly.
I've seen that outsourcing infrastructure is great to a point, but the largest users can generally get substantial cost savings by bringing their infrastructure back in house.
The cloud is great for scaling, but once you have a large dataset and more-or-less predictable growth, it could easily become more economic to handle it your self. Using something like Nimbus would make such a migration easier.
On the other hand, it's not like the S3 interface is rocket-science. Re-writing your apps file-storage interaction is the least of the effort in a multi-terabyte-migration.
