I'm not really sure how this is wildly different from the way things are now?
To me it sounds like someone who doesn't know what they're talking about describing the way servers (and even desktop PCs to some degree) are built.
If you're buying say a 1U rack mount server, there are a whole variety of configurations, your front end drive bay module and backplane can be 8x 2.5" drives or 4x 3.5", the backplane they connect to can be either SATA only or SATA/SAS, you can have a PCI-E SAS RAID card or just use onboard.
The description holds even more true for blade centers, where each bay can hold either a traditional server with CPU, RAM and local storage, or have a disk storage blade installed to be attached to other servers, as well as other options like GPUs and tape drives.
In either case you pick the parts that suit what you're building for, and in theory you could get new parts and change them around later if the workload changes.
However in reality you don't because for the majority of the type of people who are buying servers in any scenario outside of being a cloud host, you have a specific workload in mind when you're buying with a specific budget related to that workload.