In a game like this, it really doesn't matter. Just knowing the numbers are going down, and how quickly they're going down, is what is important. Knowing that something is "2 miles" away, even if I know exactly how much a mile is in real life, is meaningless if I don't know how fast I can traverse that in the game (assuming it's probably faster than a real life person could).
Equating 2 miles to 2 kilometers is probably good enough for this purpose, when precise distances don't really matter.
Take a space game, for example. knowing the enemy ship is 25,000 kilometers away is absolutely meaningless knowledge. All I know is that when I hit my boosters (which I also have no real life concept for how fast I'm actually going), I know I can get to that ship in about 10 seconds. "miles" or "kilometers" are practically meaningless in a game world where speeds and distances don't really match to real life stuff.
I'll quote myself:
But that's exactly the problem: I will once see I'm 3.5m away from my objective, remember that and see how long I take to get there.
For another quest, I'll then be 870 ft away from the objective. How the hell am I suppose to know how long I'll take (approximately)? I'll know it's closer than the 3.5m, but by how much? That's exactly why everyone is saying that metric > imperial, because you can relate one unit to the other extremely easily.
It's not dealbraking, in the end I just learned to estimate once in miles, once in feets (or what those unit translate to in FF XV, as it doesn't seem to scale), but it's kinda dumb I had to do that when the metric system is already implemented.
Or to use your space analogy: imagine that under 20.000 km, the game inexplicably decides to use a unit called shrambluckled, and jumps to 3.243. Will you know how long you'll take to travel 1.649 shrambluckled, knowing only the time for 25.000 km?
Yes, you can convert it around, but it's annoying.