I have a 256G iPhone. I think I’m using like 160G. Most stuff is just in the cloud. For an iPad it wouldn’t be any different, modulo media cached for flights. I could see some cases like people working on audio to want a bunch stored locally, but it’s probably in some kind of compressed format such that it wouldn’t matter too much.
I don't know about 'concern' necessarily, but it seems to me that 512GB for the base Pro model is a more realistic minimum. There are plenty of use cases where that amount of storage is overkill, but they're all served better by the Air, which come in the same sizes and as little as 128GB storage.
I would expect most actual users of the Pro model, now that 13 inch is available at the lower tier, would be working with photos and video. Even shooting ProRes off a pro iPhone is going to eat into 256 pretty fast.
Seems like that model exists mainly so they can charge $1500 for the one people are actually likely to get, and still say "starts at $1299".
Then again, it's Apple, and they can get away with it, so they do. My main point here is that the 256GB model is bad value compared to the equivalent Air model, because if you have any work where the extra beef is going to matter, it's going to eat right through that amount of storage pretty quick.
I think you're underestimating the number of people who go in to buy an iPad and gravitate to the Pro because it looks the coolest and sounds like a luxury thing. For those people, who are likely just going to use it for web browsing and streaming videos, the cheapest configuration is the only one they care about.
That type of buyer is a very significant % of sales for iPad pros. Despite the marketing, there are really not that many people (as a % of sales) that will be pushing these iPad's anywhere even remotely close to their computational/storage/spec limits.
What is your concern?