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I think being able to add custom stuff to your Garmin head unit is more valuable than accessing anything provided by their Connect API but that's because I don't really use Connect. I do my analysis in WKO. I'm not sure what you can do with their API as I've never had a requirement that might need it. Does it do anything amazing? Have any third parties actually used it for something useful?
Connect IQ is free, isn't it? If they don't open their website API it will simply die and everyone will be on Strava and that's that so it's their own fault.