With Zwift shifting and being able to seamlessly change gears does it make trainer difficulty obsolete? If a rider can go from 90rpm going in the flats to 90rpm going uphill (15 degrees) seamlessly it makes the grade a non issue.
It’s not exactly the same since with lower Trainer Difficulty you spend much less time thinking about shifting. I have a smart bike with 50 “gears”, and I still race with lower Trainer Difficulty because there is less to think about. I sometimes put Trainer Difficulty at 0% in order to ride very steadily, though in races that requires paying very close attention to gradient changes since other riders will surge on climbs, so it can help to get some feedback in the pedals. For me one of the other main benefits of lower Trainer Difficulty is that it increases resistance on descents so you don’t end up in a situation where you crest a climb and the resistance goes away. But if a race organizer wanted to enforce 100% Trainer Difficulty that would have much less effect on me than on most riders because I could never run out of gears.
I think one thing that is enabled that TD can’t solve for, is you likely can combine the virtual plus physical gears to get such a high gear you’ll never spin out on a steep descent. So you can mash a nice 75rpm while downhilling at 40mph