Even according to the graph above
<$99 = 19%
$99 = 15%
Entry to $100 total = 34%
Add another 20% for $149 and Entry to $150 is more than 50% of the entire market share.
True, but now you are also moving the goal posts a fair bit, from <$99 cards making up the vast majority to <$150 making up a small majority (54%).
But I'm being a bit unfair here, since your point still stands of course, i.e. that the price segment that Titan X belongs to makes up only a fraction of the total market.
With that being said though, I do believe that the original Titan sold surprisingly well, considering it's price. And given the Halo effect of such cards, chances are that this would have had a measurable knock on effect on the Nvidia brand as a whole.
Of course Titan X looks like it might have a much tougher fight in this regard, seeing as it apparently has no dedicated DP units (the rumored 50% increase in shaders and bus width, would probably eat up the 50% increase in transistor count, thus leaving no room for extra DP units). Also gaming customers, will probably be more wary of splurging for a Titan card this time around, when they know that a normal Geforce version is probably not too far off. And of course finally it looks like AMD might actually have a competitive offering this time around.