Just because you can run Server 2003 on current virtualization platforms doesn’t mean you’ll be able to do it in the future.
In the distant past, upgrades were driven not only by capability of new software, but the failure of old hardware. One of the many reasons that Server 2003 has so stubbornly remained within organizational datacenters is that Server 2003 was the first Microsoft server OS that was run heavily as a VM, and VMs don’t suffer from failure of old hardware. If hardware fails on a virtualization host, you either replace it, or you move the workloads to a new virtualization host. It’s not necessary to upgrade from Server 2003 because when you are running it in a VM, the OS is abstracted away from all the hardware.
At the moment all of the major hypervisor platforms support Server 2003 as a guest VM OS. This is unlikely to always be the case. As we’ve seen with Generation 2 VMs in Hyper-V, to note one example, there are certainly benefits to introducing new simulated hardware architectures for virtual machines. At some point, perhaps it’s a long way down the road when Server 2008 R2 has reached EOL, Hyper-V may stop supporting Generation 1 VMs. If that happened, you’d be unable to run Server 2003 on the current version of Hyper-V, and you’ll need to run an outdated version of the product to run an unsupported OS.
There’s no reason to believe that any of the other virtualization vendors will support Server 2003 in the long run either. This lack of support will take the form of not updating drivers for Server 2003 virtual machines. You might be able to run Server 2003, but the experience will be increasingly problematic.