I know of a company that has a SQL2000 instance still running in SQL7 compatibility mode.

The drive to upgrade comes when it is a physical machine with a spiralling maintenance cost attached to it. A 10 year old server will be 32bit, have a large physical footprint, require more cooling and power and be ever more difficult to supply replacement parts. In the same rack space as the old physical server machines they could host 128 virtual servers, each of greater power than the single physical machine.

There are two barriers to upgrading from SQL7/2000

1. Licencing costs

2. Huge number of DTS packages