Not counting Soviet era Aeroflot, I don't recall other aircraft of importance near that of the A380 having such a narrow customer base. (Concorde was narrower, but...)
Indeed. A narrow customer base is one sure sign of narrow market appeal. In other words, your product is addressing a small, niche market. For something as capital intensive and having a LOOOOONG development time as an airplane, small niche markets are very risky. Airbus may end up losing this bet.