The word "open" is doing a lot of work here. We can split it along several dimensions:
- Open exchange of ideas (free speech, free association)
- Open markets (private sector economy, fewer regulations, lower taxes, fewer state-owned enterprises)
- Open borders (easier and greater migration)
- "Open morality" (moral relativism, sexual freedom, drug deregulation, and general personal freedom).
I think the benefits and drawbacks of each dimension are understood differently by different political ideologies.
Recasting your first grouping of pessimist/optimist/sceptical, I think we are considering the open exchange of ideas, and whether it helps or hurts long-term stability. I personally think you have to at least consider one other dimension, open borders. As you say, much of the Russian-interference-Facebook-bad hypothesis focuses on the fragmentation and micro-targeting of demographic groups. The composition of demographic groups is directly related to border policies. Your border polices directly affect how effective this attack will be.
In the second issue, we start to hit multiple dimensions. To me, the question is, can the CCP, which only allows open markets (mostly open, anyways), out-perform the West, which allows openness in all dimensions? I don't know, and I suspect it will depend on timescale.
One of the main criticisms from traditionalists is that open morality is eating your own seed corn. You might enjoy a few generations of increased hedonic output, as you still have the residual benefits of traditional societies, plus freedom for yourself, but a few generations later, when everyone is a child-less CRISPR-enhanced furrie swiping right forever on Tinder, and the news announces that the last elementary school has closed, and the last grandma that remembers how to make a Thanksgiving turkey has died, you might feel that a terrible mistake has been made. I am not aware of a left-leaning criticism, the left seems uniformly pro-open-morality.
One of the main criticisms of open borders from nationalists is that immigrants bring their own culture, and, while assimilation occurs, it's a question of relative rates. Is the host culture (which, one assumes, created the attractive nation that people are looking to join) able to retain its beneficial qualities, or does the immigrant culture change it in such a way that those beneficial qualities are reduced or lost. This discussion often revolves around questions about "high-trust" vs. "low-trust", and tribal vs. altruistic societies. There are some cogent leftist-leaning criticisms of open borders, basically that it hurts the working-class and the viability of the welfare state, but open borders as a whole are so strongly-coded to the left than any criticism almost automatically shifts you to the right, regardless of your other beliefs.
There are also criticisms of open markets (from both left and right), basically revolving around human dignity being a primary value, and something that should not be thrown into the maw of the market optimization. The left is primarily concerned with economic inequality, and sees open markets as a perpetuation of class divides. The (male) right is primarily concerned with being turned into soulless corporate drones rather than living their true calling as shirtless virile uber-mensch, with wives that have braided hair running through wheat fields.
There are also criticisms of open exchange of ideas (currently, mostly from the idpol left), basically from a post-modern perspective, arguing that ideas themselves are weapons of systematic oppression, and that we live in conceptual prisons that prevent true emancipation for the oppressed, and that true equality requires the explicit suppression of "privileged" views, and the explicit promotion of "marginalized" views.
(EDIT: Self-identified left and right advocates, let me know if I've got something horribly wrong. I think the above is pretty fair.)