If NSA has a backdoor or has broken the encryption you are using, they will want to keep that secret for as long as possible to get the greatest possible use out of it. They will use it covertly (using parallel construction to keep it out of court cases) on important cases like (actual) national security, mafia or people running dark net marketplaces, spying on friendly and hostile governments etc.

But they won't risk it for someone with a CSAM match or anything similarly uninteresting, and local authorities won't even be aware that it's something their government can do.

Still, don't do anything illegal, but I think for most people it's legitimate to have a threat profile that assumes that encryption is effective.