The main benefits of training indoors is that it much more time efficient (just getting out of the city takes 15-20 minutes one way) and I don’t think I’d be able to do many kinds of intervals, especially longer, uninterrupted intervals near threshold or VO2max) nearby.
Outdoor rides to me complement indoor training: I can practice pacing, do relaxing Z2 rides, work on bike handling skills, etc.
I think indoor training may become “the future” of training if it hasn’t already. Or I should say the future of “most of one’s training.”
I, for one, am training exclusively indoors until I get my previous cycling weight and fitness back. This does include riding rollers for technique training.
I’ll feel more comfortable in a group setting when I’m at my fittest.
Also, where I live the roads are becoming increasingly congested and construction of new homes and businesses rampant. Training inside can be safer.
Also, I am, and always was a fair weather rider. I hate the rain and cold so even before virtual riding and smart trainers i would be on the rollers if the weather wasn’t to my liking. 
Vitamin D!
Depends on definition of “bike handling skills” but I think there is stuff you need to practice outside that goes a lot broader than just the stuff we normally think of as handling such as controlling your bike, cornering, descending, etc. Stuff that is critical to your ability to be successful in races e.g.:
This last point is quite difficult to define but I think often overlooked. Learning to be fast (as opposed to building your power) is a skill in itself.