One small nit:

“it cannot have an FPU added (that is not the case with Cyrix 486S…)”

This is mostly true but the 486SX can be paired with a math coprocessor via the 487SX, but the 487SX is actually a full CPU that disables the SX, not a full FPU!

The point is still very valid. You can't add a math coprocessor to the 486SX.

The 487SX is nothing more than a polite fiction to allow consumers (and the computer shops selling to them) to continue their existing habits of buying FPU-less systems, (because most people didn't need FPUs) safe in the knowledge they could buy the upgrade if they ever needed it.

It actually cost the motherboard vendors quite a bit more to wire up the second socket, so obviously there was demand for the flexibility.