It is growing pains. This is the necessary hashing out of common thought among any political movement. There is much upset and confusion right now and people are examining the world through different eyes. People are shedding their pre-existing worldviews. So obviously there will be infighting and bickering, as our thoughts settle and coalesce.

The broad statistical categorization of Dems and Repubs that you give doesn't express the fact that there are smaller groups within those categories who are disagreeing with each other much the same as leftists do here. (e.g. "RINOs", "never-Trumpers", the "Tea Party" of recent history) A lot of their internal struggle is kept secret, as they already have fully-fledged parties with the resources and structure to herd their members on common lines. It's not appropriate to compare open-format discussion online to the outward messaging of fully-realized parties.

As I understand, there are DSA members who are within that "Democrat" label as well who are using the reach of that party to further spread leftist messages. The statistics you list has no bearing on the fact that "leftist" ideas are spreading more rapidly in the US and people are thirsty for change. The current statistics don't say anything of the future.

When Ho Chi Minh returned to Vietnam and eventually liberated that country from capitalism, it was just him and a handful of others starting out. Different circumstances of course, but the point is things can change rapidly. Demographic statistics are irrelevant to that.

There is still much work to do. Capitalism will end one day - even if outside our lifetimes. No one now can predict how that will turn out, but it will end just as feudalism did when the conditions of the world changed.