While undoubtably true, this also presents something of a chicken-egg problem, particularly for those without free access to infinite varieties of lines. Guesstimates can be useful if only as a starting point. Well reasoned guesstimates might even provide some insight.
letting go of the magic bullet ideal is even a better bet. Good casters can cast almost anything. And when it doesn't go out perfectly, they don't make a face at the reel and blame the line, they adjust their stroke.
Spey claves are a great place to try rod/line combos.