From a practical standpoint the problem is that "required" handles a trivial subset of message validation.
I mean, I'm not going to claim that it never happens that your only constraint on a valid value of a message field is "present", but you quite often want to be able to require that one of three fields is set, or a number be between 0 and 1048576, or that a field be equal to an existing user ID, it that a string contain at least one printable non-punctuation, non-space letter.
So no matter your RPC message parsing code, you're going to need a custom bit of code in each of your handlers to say "This isn't a valid message, fuck off". Enforcing "this field should be of this primitive type" saves a lot of time, but it turns out that "this field should exist" doesn't save that much because you still have to write "... and have a sensible value" in your own code.
So you have a language feature which causes problems sometimes and doesn't really help much.
> it turns out that "this field should exist" doesn't save that much because you still have to write "... and have a sensible value" in your own code.
Disagree. Dozens of "x should exist" checks are tedious to write and even more tedious to read, obscuring the more relevant business-specific validation logic. Better to move the low-hanging fruit into the message format.
I mean, I'm not going to claim that it never happens that your only constraint on a valid value of a message field is "present", but you quite often want to be able to require that one of three fields is set, or a number be between 0 and 1048576, or that a field be equal to an existing user ID, it that a string contain at least one printable non-punctuation, non-space letter.
So no matter your RPC message parsing code, you're going to need a custom bit of code in each of your handlers to say "This isn't a valid message, fuck off". Enforcing "this field should be of this primitive type" saves a lot of time, but it turns out that "this field should exist" doesn't save that much because you still have to write "... and have a sensible value" in your own code.
So you have a language feature which causes problems sometimes and doesn't really help much.