What's ironic in this whole situation is that Strava themselves are extremely dependent on a 3rd party API: workouts from Garmin. I don't think there are public numbers on the percentage of total workout data comes from Garmin, but looking at their own status page gives an idea: https://status.strava.com/
The graphs for "Uploads" and "Uploads from Garmin Connect" are virtually identical. Seeing that, I'd be surprised if Garmin was contributing less than 75% of total uploads.
If I was invested in Strava, I'd be somewhat concerned right now that Garmin could pull a Relive on Strava...