Yes, fair question but one that I see as highly subjective. Everyone has different training history, cycling skills, priorities (life and sport), measures of success and other related motivations. People will benefit from varied levels of combinations of indoor structured training, outdoor structured training, fast-paced group rides, casual group rides, and any other activity they choose.
There are pro level triathletes that spend nearly all their time inside, and others with a nearly inverted ratio. The same is true for every level of athlete down to the casual rider looking for general health and fitness.
People new to the sport may benefit more from group riding to learn the skills and tactics necessary to compete. In some cases, those skills supersede fitness in importance to events. Others with much more skill and experience can benefit from more structured training to peak fitness in particular ways that may be difficult to attain outside.
All that is to say that people need to consider their particular experience, fitness and cycling needs and ultimate goals. There is no “perfect answer” for everyone. As such, I think we can review examples as is done here, but we need to be careful about drawing broad conclusions and applying them to a wider range than is appropriate.
The TR examples have been better developed with input from Nate, Pete and Jonathan in an effort to help with the initial question. Each one makes sense in the context of the info they provided. But even with all this additional info, these specifics only serve to prove the point that there is no single solution.
Anyone claiming that one solution (whether that be all inside, all outside, or some specific split) is right for everyone is mistaken. In the context of this discussion, trying to prove or disprove one theory with these few examples is likely to fail. Without a broad perspective to the individual situation, goals, and experience of each one studied, you may well be missing info that is relevant to the choices made and work done (or not done).
Returning to your first statement (bold in quote above), balance is most likely the “best approach”. But that balance can and will likely differ between each and every athlete. The only way to establish the related balance & distribution is to look at all I mentioned above (and likely more) then set the scheduling with that broad picture in mind.