11th is fine either way - open or closed to vehicles. I don't really see the park working out, but with some clever design, anything is possible. The local population is incredibly high so you will get a ton of use of any public space as long as it's safe, well-lit and programmed well to support children. People underestimate how many children and families live in nearby buildings.
Starting from first principles about whether or not we should go car-free, let's acknowledge that obviously all roads are useful for all modes to some degree. The question isn't that though: the question is
what modes should be prioritized where, especially when constraints exists (space or money).
The majority of downtown roads are out of balance in this regard, where car capacity and priority vastly overwhelms local movements and pedestrians, despite the highest density areas of pedestrian traffic in the city. To reset that balance to something more reflective of the existing and future pedestrian demand, it will require change on nearly every inner city road. It doesn't mean closing roads them, but it does mean trading off car capacity and priority, to allow for wider sidewalks and pedestrian priority throughout the core.
Back to the 11th Street example, it's ironically not evidence of this rebalancing, the whole thing is car-centric thinking about this idea. The only reason closure was even floated at all was 11th Street is
the least important of all crossings into the downtown for vehicles from the Beltline, not because 11th is the most important for pedestrians. Yes, the area does needs more park space, but the only reason the idea didn't die quietly and early was because the traffic volumes are relatively low and reasonably manageable whether or not 11th is open.
Car-free underpass is only an option here because even our car-biased processes and thinking can't overwhelmingly conclude we absolutely need a vehicle grade-separated crossing here given the constraints - in this case, an extremely high construction cost for a vehicle underpass. Therefore it's possible that car-free option continues to be considered.
In practice, the city centre's design flow chart works seems to work like this - essentially, you really can only get sidewalk and pedestrian improvements if the road is so incredibly unimportant for cars, you probably don't actually need the pedestrian improvements anyways:
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How it should work in the city centre is this:
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