Definitely single thread performance and storage are the main reasons not to use an old server. A 6 year old server didn't have nvme drives, so SATA SSD at best. That's a major slow down if disk is important.

Aside from that there's no reason to not use a dual socket server from 5 years ago instead of a single socket server of today. Power and reliability maybe not as good.

NVMe is just a different form factor for what's essentially a PCIe connection, and adapters are widely available to bridge these formats. Surely old servers will still support PCIe?