I've seen a few requests here with people wanting to send data to the Nextion devices in binary form (as opposed to human-readable ASCII as done now), or receive data back in ASCII instead of binary as it's done now.
I don't think one format could ever suit all users... I program in assembly language and would prefer to have communication as binary in both directions. Users programming in high-level languages might prefer ASCII in both directions.
So here's my suggestion: make the communication method programmable via a system variable. With the variable set to one particular value, communication would be binary both ways. With another value it would be ASCII both ways. With a third value (probably the default), communication would be done as it is now, for backward compatibility.