As long as you accept that leftism == irresponsibility, it's simple. Democracy punishes the responsible. You get less of what you tax, more of what you subsidize. So, as per Plato et al, democracy degenerates the population, making them increasingly irresponsible. Hence, further left, in a positive feedback loop.
Cthulu always drifts left, says Moldbug. But here at casa AI, we insist on proof.
This is a highlight of a key section of a near-future post.
Progressivism is a parasite culture. It exists to use mind control via lies to siphon off resources for the minimum of effort. But it has a problem: the irresponsible folk, who are easy to fool, are poor. Responsible folk, who are hard to fool because they into due diligence, have all the stuff. Progressives must find some way to empower the irresponsible to take the responsible folk's stuff.
Hence, democracy. Let everyone, responsible or irresponsible alike, have one equal vote. The irresponsible naturally envy the wealth of the responsible, and it's easy to partition the electorate such that the bottom half siphons off the top half. Progressives simply skim some off the current - it's not like the irresponsible are going to run the numbers and find the accounts don't line up. (The responsible have already lost, so if they run the numbers it's easy to make them look like sore losers.)
Thus, being responsible is selected against. In the new election cycle, the whole electorate has shifted to be more irresponsible - further left. However, envy is zero sum. The poor are now poorer, the rich are now poorer (except the top 0.01% or so) so it's eternally possible to partition the electorate so that the bottom half can siphon off the top half.
Badly husbanded, it collapses immediately, such as when Russia liquidated the kulaks. Properly husbanded, this siphoning can run for centuries. However, inevitably responsibility drops so low that basic infrastructure starts failing. Ten generations is a rule of thumb for the limit. The end comes at the next economic shock.