It really depends on the use case and the applications, some times it is an overkill to use a system like the RaspberyPi fo r just monitoring some sensors or controlling a relay switch. Especially when it comes to price, size and power consumption, microcontrollers are the only answer. System on a Chip devices can make great gateways for microcontrollers and supplement functionality by providing e.g., data processing, secure communication, etc.