"Trusted data" can exist in theory -- on a closed network where you control all endpoints -- but does not exist in practice (as many devices eventually connect to the Internet or exchange data with Internet-connected computers).

Therefore, it follows that re-writing parsers in memory-safe languages would provide a nice bang-for-buck.

In some cases, say the parser for an interpreter for a general purpose programming language, I would consider the data "trusted". You are already allowing execution of arbitrary code, so there is nothing to be gained by exploiting the parser.