Cubs could offer more than people think. It's just that they are going to need to offer pieces off their major league team. It's easy to forget Happ was a top 20 prospect before he was called up and I'd argue that given he's put up a 113 wRC+ last year and a 122 wRC+ this year that he's more than solidified that ranking. In fact, think you could rightly argue he's worth more now than he was then because you know he can play at the MLB level.
People are also vastly underselling the cubs pitching prospects. And I'm not even talking about De La Cruz, Alzolay and Albertos who were clearly the 3 best guys coming into the season. There's significant depth between A, A+ and AA. While it's true that outside of maybe Lange they don't really have TOR upside, most pitchers don't. Even when you're talking about guys who go in the top 15 picks of a draft it's far from a guarantee. But you look at guys like Tyler Thomas, Javier Assad, Cory Abbott, Rollie Lacy, Erich Uelmen, Jesus Camargo, Tyson Miller, Matt Swarmer, Keegan Thompson, Alex Lange, Duncan Robinson, and Michael Rucker and they are all having really fantastic years. You can make the case that every one of these guys has a legitimate shot at making the majors as a starter.