I believe the logic is fine. You seem to think you need multiple data points from the same version of the model (i.e. multiple samples per day at least) would be necessary to judge the actual performance on each particular day.

That's worse logic. How would you visualize the very large sample you would get? Even with the current 118 samples (one per day) it's already difficult to find a pattern.

Would you "average" the samples?? That would not help IMO, you would need to average the score of each image, which requires either manually doing it or finding a reliable algorithm to do it automatically, but good luck with that.

So, a sample per day which allows clearly visualizing any change in the results over months and years is a valuable thing to do and I find it hard to improve on the methodology. You just need to keep in mind that one single picture from the sample is not enough, no one is going to disagree with that... but that doesn't make it "bad logic" and it's pretty thoughtless to say so.