Most peoples data doesn't originate from Strava, right? At least not anyone I know (runners, MTB'ers, cyclists, triathletes). Nearly every single one I know uses a Garmin, Suunto, Polar or Wahoo device. Maybe a couple use a phone, but even then I don't think they record with Strava. Heck even my retired parents have a Garmin first...
Relive works with (at least) Garmin, so that's how I sync to it first, then I sync to Strava... so for me, and many many others, Strava is just a 3rd party app to Garmin. Garmin could play the strong hand and cut off Strava's access to the API and they'd suffer quite a bit too.
Disclosure: Strava member since 2011, Premium/Summit member since 2013. But, after 6 years of paying, this is turning me off. I won't be renewing Summit at the next renewal (especially since there haven't really been many useful features added over free)