These “either/or” threads are a bit silly, IMHO. This isn’t about “better/worse”, this is about looking at the tools we have available, recognizing the pros/cons that exist in all of them (inside and outside), and choosing to best suit your needs for any given purpose.
Inside training has many advantages, but also some disadvantages. The same can be said for outside training. If a person plans to do competitive riding outside, some amount of blending the two options is most likely appropriate. An all or nothing approach is overly bold and fails to capitalize on the good than can be had from both.
It’s funny to me that we see these discussions and people miss the parallel between inside riding and strength training in a gym. We don’t do squats, dips, bench and other exercises because those motions are “exactly” what we need to do. They are about stressing the body in particular ways to drive adaptations that result in a stronger body.
Insider trainer use is the same in most cases. It is about stressing our muscles and cardiovascular system to drive adaptation. Maybe people are expecting to get more from the inside training since it is a closer connection in body position on a bike and general use when compared to strength training in a gym (that is more abstract to what people are likely doing outside). I think that is OK to a point because we can gain some benefits from the closer specificity of the position and general similarity of training inside to riding outside.
But I don’t see most rational people claiming that inside training serves as a full replacement for riding outside. The pure act of riding a bike, even on the road, requires skill and that should be done outside (shocker). Add in things like group dynamics of riding and racing, or technical requirements of riding off-road, and we see all sorts of reason that we should be riding outside too.
Again, it should not be seen as an either/or. Look at the range of options and pick for your needs. Looking at what pros do with their 20+ hours of time vs the 8 or less hours that most of us have for training (and the fact that we are living “normal” lives with all that comes with them) and I see it as a fool’s errand to justify one or the other with the pro life.
They are doing what they can and need to do with the tools and time they have available. Sure, we can learn some from what they do, but we need to take a wider look at the similarities and differences in their training/lives and those of our own.