in theory, perhaps, in reality middle class will disappear not becuase shit will be evenly distributed amongst all the poor, but because with time the upper class will get wealthier and stronger and will squeeze middle class out of existinceWell actually both will happen; communism will not only make everyone poor, but will also protect the select "few" (politburo, kom partiya, etc.) from the miserable masses, just like in "raw" capitalism with a weak/non-existent gov't, the "rich" will push thru laws that will benefit them disproportionately (anti-competition, etc.). Only difference is that in a "raw" capitalism you are referring to, everyone (rich/powerful and poor/underprivileged) will be somewhat better off (*) due to inherent advantages of free markets over centrally planned economy.
The reason it's difficult for the middle class to survive is simple: the middle class works to make money, and doesn't have the time (like the lazy) or the money (like the rich) to fight politically. Middle class usually wins by accident, when a 1) honest and 2) competent president happens to come to power (in USA, that is). It is a rare combination and usually, the 1) lying and/or 2) stupid presidents make things gradually worse until the country wins the lottery again.
(*) "Somewhat better off" means even politburo can not go to 6-star resorts that super-rich capitalists can go to, and the "poor", as I showed in my calculation, could live more or less OK (given level of technological progress 100 years ago) if they had a job - the famous "barracks" where New York factory workers lived are not much worse than the sharing "Hrushevka" apartments like in post-war USSR (we're omitting Stalin's mass murders now, to concentrate on economical differences).