It shouldn't be expensive to make an I2C smart-switch that allows you to place as many buttons as you want on a single set of 3 wires (or 4 if you want a simpler circuit).
Hum, it's terminated with resistors on both sides, so the boxes need power. That makes it 4 wires.
I imagine there's very little difference on cost between 3 or 4 wires. But the number of nodes described is not really compatible with making each button its own node.
