To invoke the old "Set your default programs" interface in 1709: 

Run:

control /name Microsoft.DefaultPrograms /page pageDefaultProgram

To invoke the old "Associate a file type or protocol with a program" interface in 1709: 

Run:

control /name Microsoft.DefaultPrograms /page pageFileAssoc

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Just in case other Control Panel applets have this happen, you can find the various page names using the Windows Dev Center's list of Canonical Names of Control Panel Items:

https://msdn.microsoft.com/en-us/library/windows/desktop/ee330741(v=vs.85).aspx