Java/Go/C++ are perfectly capable of parsing a "type" key and conditionally parsing different shaped data. If you make a programming mistake here, you'll get a parse error (bad, but not a security problem). The pushback seems to be that a Java/Go/C++ implementation adds LOCS and won't gain much by doing this extra step so lets make the protocol itself match match their (less precise) data representation.

FWIW there is work towards improving Java in this regard: https://cr.openjdk.java.net/~briangoetz/amber/pattern-match....