For serious athletes, Strava offers quite a lot and segment competition is a very small thing. I'd go as far as to argue that for this slice of customers, segments are just for fun and there are other real reasons to use it. Here's a quick overview of the things I find valuable that I wouldn't want to go without:
- Big community, with automatic ride grouping (showing rides together in the feed when you ride with other people). Ability to social network by leaving comments, adding photos, writing a story and so on.
- In depth (although I would love some more) ability to analyze your own performance: plot comparisons between segment rides by time; compare your power curves (W / duration) across different time spans; track your running PRs (1k, 1mi, 5k, 10k, 1/2 and full marathons);
- Calculate TSS (training stress score) per ride and per week in order to track training stress. Also supports tracking weekly duration, distance and so on. This is done based on HR during the exercise, I believe there are well known formulas. - Integration with a bunch of external services. I want the rides I track with my Garmin devices that end up in Garmin Connect to automatically end up in Strava. There's a thing called tapiriik (https://tapiriik.com/) that could be helpful here, it's also open source.
- Very nice route planner. There are other services for that (e.g. Komoot), but Strava is able to incorporate their data trove to make smart routings. When I make a route in Strava it also ends up on my Garmin devices, which is extra nice. No transferring GPX files left and right. The fact that it's built-in, easy to use and "right there" makes it quite an attractive option.
- Nice API. I hate that Strava is adding more and more limitations to it, but some of the local cycling groups use it to organize group rides and it works quite well. I believe any competitors will need to outdo Strava in terms of API coverage and limitations.
- Fitness & Freshness graphs, ability to create training plans, ride cropping, exploration ... many other things I don't use daily, but they're just there.
One should also probably read up on previous efforts in that sphere. Endomondo used to be pretty big, but then kinda just fizzled out. There's Ride with GPS and a bunch of others, but nothing on the scale of Strava. Why? I don't have a clear answer, it's probably a multitude of factors, even product timing.
As a daily user (7 workouts / week), group ride organizer and outdoor person, I'd be happy to discuss Strava and its alternatives further.