Pogo pins are expensive. Somewhere around $0.15-$1 per pin. This design has four pairs of two pins. So just the connectors are over a dollar per key, possibly multiple dollars
That's fair, I didn't take that into account. I wonder if some alternative design that doesn't use them is possible
Inductive coupling could work for both power and data, but probably not at high enough power levels to have LEDs per key if you want to go through 10+ keys in a row...