I guess you're probably joking, but Iwan Rheon is a better actor than Finn Jones and IMO Sophie Turner by a factor of several thousand. I must confess when I saw Finn Jones cast as Iron Fist I let out an incredulous "Whaaaaatt". (I haven't seen Iron Fist, mind you, maybe he's amazing in it ) I haven't seen the IMAX Inhumans, naturally, I'll wait for it on TV like most people, but from the clips Rheon looks OK in it.
Not that there's a curse, but it definitely doesn't apply to actors over Iwan Rheon's age (an-older-than-you-might-expect 32) as aside from Dinklage in Days of Future Past, Game of Thrones didn't stop Lena Headey from appearing in the excellent Dredd.