Basically what we're saying is that better options already exist right now than what SC is going to become - with reasonable estimations based on the current state of things, their previous progress and their announced plans.
SC's scope covers gameplay from a lot of existing game genres, and although I wouldn't reasonably expect a game of this large a scope (even though CIG and the fans do) to have the best gameplay ever on each and every of its many parts, I would at least expect and require each part to be at least decent and vaguely enjoyable when taken separately. A reasonable estimation of SC's future wouldn't even bring the quality of these parts close to mediocre, let alone good.
For example, comparing SQ42 to FSO and SSZ is relevant. Even in the best of circumstances I wouldn't expect SQ42 taken on its own to surpass either of those, because SQ42 is part of a larger project and the interactions between the different parts implies sacrifices to make the whole game work - but I would expect it to be decent and somewhat enjoyable on its own. However, in the reasonable expectations of SC we have laid out before, SQ42 is looking to turn out horrendous, both in term of gameplay quality, depth and storytelling. The same can be said for each of the parts SC is made of (persistent universe, FPS, etc).
I hope this clarifies things. Basically even by lowering our standards to something coherent with the compromises a game with this large a scope will always be forced to do, we just can't reasonably expect - based on what we know and what we can expect to happen in the future - any part of SC to reach acceptable levels of quality and enjoyability. Let alone be the Best Damn Game Ever.
And that is comparing SC's parts with _existing_ games, not the games that will be released by the time SC comes out, assuming it does.