Most practitioners do not have the mastery you're talking about over any language at all. I can't support normative advice to new entrants to the field based on a whim we might have about how current practitioners should behave; I have to look at how they're actually operating.
For what it's worth, I came up as a C programmer. I value the experience, but do not value any sense of mastery I might have ever felt I had over all the minutiae of the language.
Most practitioners can write FORTRAN in any language, i.e. they fully master FORTRAN-level language features. The interesting challenge is to push that a bit higher.
For what it's worth, I came up as a C programmer. I value the experience, but do not value any sense of mastery I might have ever felt I had over all the minutiae of the language.