The notion that you can just go out and get shortstop prospects like Didi without giving up another significant part (top 5 prospect) is absurd; if Didi costs something less than that, there's probably something wrong with him. .
Couple of points. In addition to Didi, he Reds gave up Drew Stubbs. He was coming off a .680 OPS and 100 SB over the previous 3 years and was and is a starting CF. Choo was a rental. And Bauer while highly ranked, refused to tinker with his delivery, which the D-Backs demanded and he refused. These things were all factors in this complicated trade, alone with 6 other players being involved. Since the trade, Bauer has 33 IP at the big show with a 5.67 ERA. Last year he had a 4.15 ERA in AAA. Maybe not tinkering with his delivery has hurt his results?
Lastly, I think you mean top five prospect for a team. Not a top five overall prospect. By which I would say that each teams top five prospects are not all created equally. So a package that starts around our say 7th or 8th best prospect would not be absurd. Trevor May was the Phillies top prospect and I think he slotted in at 7 or 8 immediately after the trade.