I think the problem is the realtime. You can certainly write software that emulates a microcontroller on the Pi, but the problem is that it can't guarantee tim

I think the problem is the realtime. You can certainly write software that emulates a microcontroller on the Pi, but the problem is that it can't guarantee timing, since it's running on Linux fundamentally, and other processes can pre-empt it. For certain things it doesn't matter as much, such as for a door lock or something like that. Then you gotta worry about power consumption, since running cables to your lock is problematic, and it has to run on batteries for months at a time. That's a reason why realtime controllers can't run on Linux - if you were controlling an industrial robot that controls welding temperatures and times, even like 300 ms off can drastically affect the finished product with fit and finish and metal heat treating. "This whole batch of parts worth hundreds of thousands of dollars went bad!" "Well, the network is down, and the robot controller was busy trying to reconnect to the network, so that's why the welds were formed over 300 ms more than they should have been." That can't cut it in industrial applications.