Does it count as the five stages of grief when you never leave denial?
Anyway, if Hillary died or had to pull out for health reasons (since let's be honest, that's the only way she isn't the nominee), Dems would still be under tremendous pressure to nominate a woman after all the hype about the first female president. Women also tend to make up 55-60% of the Democratic primary electorate. It's not a group they can exactly afford to alienate, particularly after the near miss in 2008. So in that sense the R-NY guy is correct. In 2016, it is extremely unlikely the nominee will be a man (though I don't think this applies indefinitely for the future like he does).
Anyway, of these options, I'd say Gillibrand would be strongest.