The other thing to consider is that data usage is going up constantly for certain apps, even with the same time on app. Ie streaming YouTube videos in 1440p vs 720p because you phone automatically matches the screen resolution or Instagram introducing longer videos and stories compared to just photos. I've used the same 4 or 5 data heavy apps for the last few years and my data usage has doubled, but my time in each app has stayed the same.
So if I can pay $10 a month to get unlimited YouTube, I can a) cut my data usage and phone bill in half and b) effectively hedge myself against constantly increasing data usage (ie switching to 4k streaming, YouTube updates bitrates on other resolutions etc).
Edit - I'm sure someone has pointed out something similar somewhere in this post
