Not that Marc Andreessen is necessarily /right/, but the argument that the middle class "definitely exists" is, like any statement in sociology, rather difficult to prove.
In fact, "middle class" means different things in different places. In Britain, "middle class" generally refers to doctors, lawyers, bankers, senior government officials and many entrepreneurs. The higher rank "upper class" is historically reserved for genuine aristocrats - if you're not a Lord, Viscount, Baronet etc. then you're not upper class, no matter how wealthy you are. Richard Branson is "middle class" by this definition. Essentially, British middle-classness is about values rather than economic status; even a poor person can be middle class if they listen to Radio 4. (I simplify, but not by much).
Marxists (again, simplifying) generally describe the "middle class" as being a fairly narrow band of people who benefit from capitalism by virtue of occupying privileged positions - bankers, CEOs and so on - without actually being capitalists (owners of capital) themselves. In Marxist analysis, this middle class is effectively bribed to support capitalism by being rewarded with power over their fellow workers, but this power is always exercised on behalf of the capitalists. Weirdly, this idea of a narrow middle class of functionaries acting in close concert with [venture] capitalists is pretty close to the Andreessen world view as described in the OP!
If we accept the mainstream American definition of middle class as being about income levels, then Andreessen can still be correct if we read him as saying that the middle class does not /inevitably/ exist, or is not /inevitably/ as large as it is now. Falling median income in the US could, if continued, result in the eventual shrinkage of the middle class, or the redefinition of 'middle class' to include poorer people.