The "reform is impossible" is a self-fulfilling prophesy because it leads leftists to never try to get involved in the party, which means they'll never get a seat at the table, which means they'll never be able to steer the party.

I certainly can't prove that the influence of big money can ever be overcome within the party by grassroots organization, but you also can't prove that it's impossible (you can only prove that it's difficult, which is something I certainly won't dispute).

You certainly can't prove that a true socialist movement will ever gain traction in America. It seems like the general public is so brainwashed they would rather be indentured servants of large corporations than lift a single finger to seize the means of production.

So we're left with two unprovable paths to consider, and here's the thing: the two paths are not mutually exclusive. Leftists can try both at the same time with neither being disruptive to the other. So this is the pragmatism: consider all possibilities and put the eggs into more than one basket.