will  PLC still be required in nearly future in process control  if DCS can be more cheaper or has small size function?

I think the lines have blurred so much with regards to functionality that it may not matter what the platform is labeled as (DCS, PLC, PAC). All have I/O, logic processing, networking, & HMI...even previously "differentiated" features like distributed processing (local PID etc.).

Maybe your question should be - Will DCS still be required in nearly future in process control if PLC is more cheaper or can grow from small to large size function??

It doesn't really matter what we call the controller, in fact let's just call it a controller and be done with it.

PLC, DCS, PACs are overlapping term with many controllers available today with the functions of many of these.

The key is how you program the device and what communications capabilities it has.

Electrical engineers and mechanical engineers will always want ladder logic because it is easy to understand. Communications is to discrete devices mainly to keep cabling costs down.

Process engineers will always want function block diagram because its what they know. Communications include HART, Fieldbus Foundation to intelligent devices to facilitate setup and maintenance.

And the next generation of engineers will be happy programming in structured text but also C, Java, maybe Python, because that's what they know. Communications will be transparent over Ethernet with shared memory and object based protocols replacing what we use today.