Advocates of direct democracy envision a system where each person's vote has an equal power. The narrative goes along the lines of "equal rights" and that each human being has an equal weight to society. This might sound logical but it is neither practical or realistic.
Humans do not value each other the same way. If we did, we would all date people with no respect to looks, income, social status or behaviour. We would pick jobs, books or movies with no respect to their quality. In a place where everyone is equal, no one truly matters. The complementary argument that "everyone should instead be treated equally" still does not apply because simply we don't even bother to engage with those that we do not consider to have value in our lives. In practical means, this is the same as rejection.
We cannot vote for things that do not offer value to us. We cannot appreciate value for things that are outside our scope of wellbeing. This is what most democratic and consensus systems get wrong. People are too unique and as we progress technologically we add more qualities and needs in our repertoire. We cannot possibly vote for things that fall off our radars nor can we agree on a large set of values. Not only is impractical but also pointless. We are too selfish and will remain as such because this what our nature dictates. We value ourselves, then our family, then our kin with our monkeysphere getting weaker as we increase the numbers.
Modern democratic systems assume that we all live in one planet and somehow we should all be coordinated. This is false and largely unscientific. Any sensible zoologist will tell you that each part of the planet has a different ecosystem with unique characteristics. African tigers cannot be compared to Tibetan tigers because they have different needs even if they look similar. Similarly, humans have developed such unique needs that in the future even medication will be composed to match our special physiology. We just don't do it because we are not there yet technologically.
Politically though, we are doing it to a small degree. Switzerland is a prime example of how immediate consensus can bring people together to decide on small things. Small blockchain communities and networks also seem to work well. The problems begins when the network expands and the votes start be skewed. The direct value and weight for each person gets nested under unrelated values that are skewed massively in context. This is what we witness on facebook or twitter. A negative review can go viral as much as a positive one. The virality though does not represent the true impact of the event but rather a random sensationalist flare that will die out soon enough and nobody will really remember. The value something has is dependent heavily on our given mood at the given point in time in relationship to common psychological tricks that other humans are also susceptible. Pretty much, monkeys jumping inside a cage at the sight of a moderately large banana.

I don't believe the current "upvote/downvote" system will survive due to this group size limitation. There are no boundaries in networks unless the communities set them. In a world where everyone belongs everywhere, network boundaries mean nothing. Surely, small groups work best with consensus. This is how small villages develop great relationships and assist each other while as we go bigger into cities the needs (and value) is spread across the entire fabric. Everyone ends up expecting something the need from everyone else that might not even care.
As long as we are humans with a need to belong and be accepted to more and more people we will never have extended successful consensus systems. We might think that it works because the numbers will present visible comparisons in the millions but in reality we will care as much for those numbers as much as we care about individual sand particles at the beach. This is also the reason why we get so emotional about a baby that has suffered and made the news but don't give a shit about the 16,000 kids that die everyday from hunger.