If a raid is only attempted, let alone completed, by a small portion of the population, can it still be worth it?
Let's start with an assumption: players who are in a raid are still playing because of that raid. This is obviously a terrible assumption, but if the physicists get to assume a spherical cow in a uniform magnetic field, then I get to assume this. Based on this, a raid that is only seen by 1% of players is only retaining 1% of players. But I doubt that is the entirety of the effect.
There can be the opposite effect, if an inaccessible raid driving players away. This is not the effect of them running out of content or burning out, but of them receiving a message, real or not, that they are not wanted. While otherwise happy with the game, they get the idea that the game is not for them, and they are driven away by it.
Even that effect may be opposed as well, with players who, while they do not, and possibly will not, see the 1% content, are motivated by it. They keep playing because of it, to get to it, even if they do not actually get to it.
The second and third groups are what I'm interested in. They are both groups which do not directly experience content, but are nevertheless affected by it, psychologically. Between the two of them, plus the direct 1%, is there a net gain of subscribers?
There is another factor that I've left out, the budget, which could shift the equation. If the 1% content is consuming too many resources, it may deprive the second group of content, making real their belief that the game is not for them.
It is a simple fact, that "x% of players did y content", but by itself it is not particularly useful. We would also need to know, if players stay despite not using that content, because of the content, or if even more leave because of not using that content.