GT1 wasn't great just because it had lots of cars, though. It was the quality of those cars, the accuracy to which they were modeled and the unique driving dynamics for every one. The radical structure of the career mode. The depth in upgrading and tuning. The number of tracks, and the brilliant track design. 150 cars was groundbreaking at the time and surely an exciting selling point, don't get me wrong. But without the other pieces of that puzzle, it wouldn't have mattered how many cars the game had.
Likewise, I don't think GT Sport will live or die on its car count. Whether it's fantastic or terrible, it will be for other reasons. Game development is so different than it was 20 years ago, and the problems developers face are different. Content takes exponentially longer to produce. It's unrealistic to assume the number will continue to climb and climb just because the years do.