Configuration is kind of joke. 16GB RAM and zero free slots on high end desktop? My PC had the same price three years ago, but it has 5GHz CPU, 32GB RAM, 1TB on SSDs and 5 TB on spin HDDs.
The slots thing is a design decision. Apple doesn't think people need them, I think they may be right. The Thunderbolt connectors are direct to CPU , not even the north-bridge in the way. I remember when they said people wouldn't need ADB or SCSI. Or floppies. Or serial ports. It worked out. If your 5 GHz CPU a Xeon with ECC? The RAM can be upgraded (those are options), the SSD can be upgraded (1 TB is an option) and…
I am more inclined to say Apple doesn't want people to think they need them.