I'm not a big believer in mandates, but requiring green roofs on new buildings can be made to work financially, if not physically (in Denver's arid climate).  My big opposition is expecting the owners of existing buildings to absorb the cost of structurally reinforcing their roofs to safely carry the added weight of soil, water and vegetation that green roofs require (since they were never designed for those kinds of loads), when re-roofing is required.  And as the EPA has discovered with their building, what's really needed to be successful is old-school sod roofs, not the new, lightweight, "high-tech" green roof products in trays that advocates are claiming will work great for retrofit situations:  https://www.epa.gov/sites/production/files/2017-09/documents/denver_case_study_508.pdf