A serial interface can run a lot faster than anyone can type, eg i2c high speed is max 3.2Mbit/s.

SPI can be way faster than that I believe.

true, but adding another link (microcontroller) in the chain is going to add delay anyway. I think the original suggestion was pointless: instead of adding more microcontrollers you can just replace the main uC with the one that has enough pins. The reason this is not done is uCs with >100 pins are usually more powerful and expensive, so you can't just pay for more pins -- you also have to pay for more processing power and features you don't need.

Depends on the speed/latency of the interface. Although you could probably say the same about the scanning matrix too.