I guess I'll try this again.  There is no requirement in the Initiative to add load beyond code standards or what the roofs are designed to support, or to require roofs to structurally retrofit as part of a re-roof.  If a roof can't bear the load, it'll use an alternative compliance path or be exempted. 

The planter loads are entirely calculable, and those calculations are a required part of any permit (just like any kind of roof-top planter or rooftop addition, both of which are entirely common.  There is no potential for the catastrophes you warn against, and there is no history among the millions of square feet of green roof installed annually that supports that fear.

The Initiative also mandates the creation of a Mayor-appointed task force to modify the resulting ordinance as necessary/appropriate.  If there are any unintended consequences anticipated or discovered,that Task Force is specifically established in a manner that can quickly address them.