> Your criticism is easy to make with 20 years of hindsight.

True. I never claimed otherwise.
This does not make my criticism invalid.

> Modbus is a standard that both simple and adequate.

Simple - True.
Adequate - False if you cannot tolerate its flaws.

I would not get in a plane that used Modbus for fly by wire.
I would not bet millions of dollars automating a factory with it.
If it were adequate, nobody would bother designing anything else.

> Profibus didn't come until 20 years later and is more complex.

True, that's because designing a robust protocol (that has mechanisms to avoid
uncomfortable worst-case and  what if' scenarios) is a complex problem.
A good spec goes into all the details so that everyone designing for it
does not have to provide them or worse not work them out at all.

> Isn't the Modbus packet start character ':' a unique character?

True - in ASCII mode
False - in Binary mode and your data payload can contain this byte

>  No protocol, will allow a slave device to wake up in the middle of a packet
> and properly handle what it didn't receive.

True - I never claimed it should.

A protocol should be able to assume the start character will never appear
in the middle of a packet. For example, X25 uses bit-stuffing to ensure this.

Arcom Control Systems Ltd wrote a booklet entitled
"Everything you need to know about Modbus but were afraid to ask"
describing all its foibles and how people usually worked round them.

Hope this helps!