These lines are still popular because they offer something a rack of Dells can't - a nicely packaged solution of extremely reliable (and powerful, no pun intended in the case of i) hardware and software, exquisitely refined over many decades.
There are, and it isn’t a new thing - California Software Products came out with BABY/400 in the early 90s, to support porting RPG apps from AS/400 to DOS or OS/2-and Windows and NetWare versions were soon added as well.
But the problem all these solutions have - IBM i has so many gazillions of features, many of them legacy (sometimes with multiple different ways to do the same thing, just for historical reasons), none of these packages can implement it all, they can only implement a reasonably common subset. And if you are really lucky, your app only uses that subset, and it all just works. And if you are a bit less lucky, it only uses one or two extra bits that are easily replaced, and the rework required is modest. And if you are really unlucky - and the bigger your app and the more technical debt it has, the more likely it is - the bits missing may be so significant, using one of these solutions might not be viable.
Looks like an excellent opportunity for an open-source project to step in. Would need a lot of AS/400 and IBM i hobbyists to cover everything.