Then there is nothing but ones and zeroes, and nothing to compare, and no insight to be had.
Yet, C++, D, and Rust are much more like one another than any of them is like any other language. Your Turing Completeness tells you less than nothing about any important distinction between them.
Once you have Turing completeness you have the full set of possible programs, end of story. Languages aren't about changing the set of possible programs, they're about making good ones easier to write, and bad ones harder.