I have a 16GB iPad 1 still going strong - I barely use any apps outside of the ones preloaded [exceptions being Twitter, Facebook + Kindle].
When I travel, I load on a few movies / tv eps and I watch those. When I travel again, I just swap them out. I do the same at home. 16GB is ample if you're happy to swap content on & off your device.
16GB is enough for my books and enough movies (compressed appropriately) to keep me entertained on a transcontinental flight.
I have a large music collection, but that typically lives on my phone, not my ipad.
Of the machines I own, internal storage is just cache. I have a 9TB zfs NAS for stuff I actually care about. Whether an iPad has 16GB or 128GB of storage doesn't really matter; they are both equally inadequate. There would need to be two orders of magnitude increase to fit all my stuff in.
My iTunes folder is currently at 60+ GB, and that's after purging all the TV shows off of it.