Imo, this is disingenuous. GT7 was not a step back for the series when compared to GT Sport. If there was any game that was a "step back" for the series, it was GT Sport since that game was lacking revolutionary features from previous games in the series that "advanced the series".
When GT5 and 6 introduced dynamic weather and time of day transitions, GT Sport was missing all of that. When GT5 and 6 added course creator and a more robust B-spec mode, GT Sport was missing all of that. GT Sport also had significantly less cars and variety than most of the other games in the series. GT Sport also launched without any semblance of a proper singleplayer mode, which turned away a lot of people who were not interested in Sport mode, or online in general. Even with smaller singleplayer content patched in later, it's still the weakest in the series. GT Sport's tuning aspect was also much weaker than GT7's. There's so much more to mention, but I'm sure you get the point by now.
While Sport mode and VR were cool additions to the series to be fair, what the series lost in exchange for those 2 things was not worth it.
Now, you compare all of that to GT7. GT7 has all of those features GT Sport is missing, baring B-spec and course creator. GT7 advanced the series by having the best dynamic weather and time of day features and physics simulation in the series to date, even factoring in so much minutia such as drying line strategies. The overall driving experience and physics simulation in GT7 is the series's best to date. And, say what you will about GT7's Cafe mode/singleplayer when compared to older GT games, at least it was there on release, and it is far better than Sport's offering, along with missions mode. While this hasn't happened yet obviously, GT7 will likely get VR support after PSVR2 launches, and if it does, it'll likely be much less limiting than GT Sport's.
While GT7 may not have been a large leap in advancement compared to games like GT5 when speaking of just raw features, it still pushed the series forward and was most certainly not a step back.
It's fine if you have differing opinions on which GT you like more or less, but saying that GT7 was a step backward for the series, when ignoring all of the glaring faults GT Sport has is very disingenuous in my honest opinion. At least GT7 is feature complete.