Back to the main point of this discussion, I think this essentially leans towards a “clinical vs organic” type of discussion. ERG mode being the “clinical” equivalent, and Resistance mode being the “organic” equivalent.
For building a proper and complete training program, I believe a person should include both. The ERG is great because you can carefully control the effort to not exceed or fall short of the prescription in the workout. It’s not perfect, but it likely applies a more controlled and steady load to the rider.
I say this even though I know that many can do a great job of hitting numbers in Resistance mode (via smart or dumb trainers. But that level of precision on Resistance takes some serious attention and a well known use of the bike, trainer and gearing. As such, it requires more of the “cognitive load” as mentioned above.
This can be good or bad. It will be good in testing the rider up to the point that the attention required leads to limits of the performance. This is one reason ERG is useful. It relieves the rider from having to pay excessive attention to the power target. There is still a requirement of the rider, to hold a proper and consistent cadence, but that is relatively easy in comparison to watching power targets in some cases.
As with everything in this world, there are no absolutes. I think the idea should be to look at the pros/cons of each tool at our disposal, and use them appropriately to take the most from their advantages. As such, I spend a lot of time in ERG mode because it is great from a pure training perspective.
But I also spend time doing “free” rides workouts in Resistance or Simulation mode, to make sure I keep some of the “outside” feeling in my training. I found over the years that a mix is well worth the effort rather than sticking to one particular direction. Variety does us well in so many aspects of life, and training is no different.