Are there any downsides to sharing a successful proposal with others?
Funded NSF proposals are requestable directly from NSF through the Freedom of Information Act. Proposal authors are given the opportunity to suggest redactions to NSF of proprietary or other information that should not be disclosed generally, but their FOIA office has final say on what gets sent. This kind of information might include not yet released product specs that would be bought with the grant funding or salaries of non-public employees. I suspect that all non-classified US federal agency grants are FOIAable through a similar process, but I only have FOIA experience with NSF.

Some people put their successful grants online, some people keep them secret. Your special sauce, in the end, is probably not that special, but good writing and good organization may be informative to weaker writers in the world. We don't publish our grant application texts online, but they're definitely out there due to FOIA.