Interesting thread. I use a smart trainer for indoor structured training and outdoor riding for endurance, group rides, mtb etc. Plus target events of course.
They all compliment each other perfectly. My bike handling skills don’t vanish after indoor training sessions. I can often get a higher quality workout from riding indoors. My local terrain lacks long sustained climbs, so I often simulate alpine climbs on the trainer with accurate grade simulation, so power and cadence are both realistic. I lift the front wheel to get the right position too. It works for me when I go back outside and doesn’t feel a whole lot different.
Outdoor winter road riding where I live is too dangerous for my liking. I would rather maintain my fitness indoor with the odd outdoor ride on brighter days. In spring I come out of the box fully fit. I don’t think I could achieve that from exclusively riding outdoors unless I moved to southern Spain.