In Marxist dialectics, it is important to tolerate a sort of fuzzy middle ground between opposites (in this case the proletariat and the middle class). Here, the extreme ends of stock/home ownership land people solidly in either class, it would be ridiculous to claim that owning any stock at all makes you middle class, or that someone making most of their money from stocks is proletarian, but the middle is more unclear.

For most purposes an exact line does not need to be drawn because the object of study is not an individual, and doing that would introduce a seed of idealism. Around what part of the spectrum our vague idea of a line is located depends on our reason for doing the analysis.

For example, if we look at the evolution of American ideology, it would be fair to claim that owning significant assets makes people more receptive to middle-class and bourgeois ideas, and owning more probably makes you more receptive. Here it would likely be useful to call people owning significant assets middle class. On the other hand, if a party has a quota of a proportion of members who must be proletarian, it does not make sense to reject people based on the value of their home, so the same people who might have earlier been middle class are now, for this different purpose, considered proletarian.