The friction of inner city life is what often leads people to leave, or not come in the first place. Gets too pricey, gets too small, parking is too hard, traffic is too busy etc. This would be one of those decisions that needlessly adds friction. I think it's fine to prioritize pedestrians and bikes in the Beltline (just like its fine to prioritize cars in the burbs), but I just don't see any viable reason to get rid of the car lanes here. The "alternate" proposal does everything well, while still clearly giving a much improved preference to bikes and pedestrians. Getting rid of the car lanes, for a very dubious underpass park, is just flat out dumb.
Honestly, for me it just comes down to thinking that having options is good! Just like many great cities have a wide option of housing styles and budgets, a great city allows for multitude of travel modes that gives a wide cross-section of users (residents, visitors, businesses etc) to chose what fits their purpose. I think we all appreciate having the option to travel via whatever is most efficient at the time...whether it's biking, walking, transit or driving. Usually the people who want to force people into this car-free utopia are in their 20s and/or have no kids. They typically don't yet appreciate that priorities change as you start a family, or you have a business, or you get older. It doesn't mean that being car-free is a bad way to live...it just means that for many people, that's just not realistic or desirable.
Choice good!