The phased approach using data trends is the method the White House has developed and is now encouraging - and I think many states, cities, and organizations using it to some degree.
I have needed to look at the White House plan to understand the re-opening plan for a large organization that I do legal work for . . . I have only been asked to spot (tort) liability concerns but I have reviewed the plans and they're based on (1) requiring case and test trends to hit certain metrics to get through the "gate" in to the next phase; (2) continued use of some mitigation measures - some in a sliding scale of impact - to protect the vulnerable groups that include the elderly and healthcare workers, and (3) monitoring the situation as a function of health-care capacity.
The New Orleans plan public website doesn't state the specific metrics (there probably is a more detailed plan that isn't on the site), but the White House plan includes, for example, that phase 1 cannot begin until there is a downward trajectory of documented cases within a 14-day period or a downward trajectory of positive tests as a percent of total tests within a 14-day period (flat or increasing volume of tests). And phase 1 is a limited opening, it's not a immediate return with a few casual measures still around.
I didn't watch her press conference but is that where you're getting the "zero new infections" baseline that you're claiming? How is the New Orleans plan different from the White House guidance?
https://ready.nola.gov/incident/coronavirus/mayor-cantrell-outlines-steps-toward-reopening-(1)
https://www.whitehouse.gov/openingamerica/#criteria