Question:  Can we replace a PLC with an Arduino in industrial applications? Both got Digital/Analog IOs, user program and a scanning cycle. Clarify your answer.

Answer: No, But before we discuss the reason of Arduino's invalidity for the industrial automation applications, there are some essential information should be known about the Arduino platform, that the Arduino is kinda categorized as a DIY or HobbyKit open source environment for creating interactive electronic objects, small scale projects, lab applications and simple home automation applications.

Straightforward Reasons:
1- Limited I/Os, while an industrial system may have hundreds of I/Os.
2- Using the LAD or even STL languages are more familiar to electrical engineers rather than the C++ which is more related to the ECE engineers.
3- Arduino is not designed for harsh industrial environment conditions such as high temperatures and continuous operation.
4- PLC firmwares are designed for certain expected errors/critical failure situations related to the process field which are easy to avoid/detect using some OBs and SFCs which will take much longer time and effort to be implemented using C++ and Arduino. 
5- PLC systems are also designed for integral industrial systems, In other words it is designed and manufactured specially for these applications.
6- Maintenance, Service, System Modifications and Error Detection are way easier in the PLC rather than tha Arduino.
7- More further, In PLC scan cycle all the inputs and outputs are being checked in the same time ( Reading PII and Writing PIQ), but in Arduino they are being checked based on line order of the program code.
8- PLCs are already designed to deal with the Industrial Application Communication Protocols(PPI, MPI, Profibus, Profinet etc.)
9- I guess the I/O cards/Signal Modules of the PLCs can withstand more current levels than the Arduino can do.

Please, Correct me if i'm wrong at any of these points.