This has always been my view. And it doesn't change if the "test" becomes an informal evaluation over time. And it doesn't change if (in either of those cases) the belt doesn't even exist.
Well in some styles they do promote you based on your evaluation over time as opposed to formally testing you. From my experience that's how its done in BJJ, at least that's how its done where I do BJJ. As for the belt not existing, there are some styles that use other symbols to display rank, for instance, some styles use patches.