Capital might allow UBI below subsistence, and this very well could happen to stave off crisis. It just can't be high enough that people aren't incentivized to sell their labor power (be a wage-slave). This is arguably what minimum wage and tax credits already are.
cqst
There are definitely "right-wing" anarchist currents that are popular and not as unhinged as how "anarcho-capitalism" is generally characterized, as in, anarchism that espouses the use of capitalism, but I have not really seen any genuinely "socially conservative", anarchists. Pacifism is reactionary and emboldens the status quo.
The term leftist is considered to come from the fact that people sitting on the "Left" in the Estates General in France. Generally, liberals, (Jacobins, Third Estate). There is very poor sourcing for this online, but Wikipedia cites some untranslated difficult to source french book.
In mainstream discourse, "left" takes on a number of meanings, but to the extent that left is meant to mean anti-capitalist, and/or, Marxist, anarchism has left wing currents, hence, left wing anarchism.
Extrapolating out "human nature" from past human behavior is a form of surface level inductive reasoning that does not hold up to scrutiny. Inductive reasoning is flawed, "humans are naturally hierarchical" isn't an argument. It's possible to use material analysis to determine the source of hierarchy and as humans we have the ability to change our material conditions. Also ignoring anarchist projects that succeeded in horizontalism, maybe not in militarism though.