This is my 10-15 years from now opinion. It may surprise you, it may not.
Daniel Radcliffe. Durable. His career arc has followed the typical British actor's career: dabbling in theatre, movies, and TV, from the experimental to the mainstream. I think this works in his favor, as it means people will stop associating him solely with Harry Potter. I think there's a 50/50 chance he'll become the Colin Firth of his generation (iconic for one role, but getting esteem and awards for other roles). Alternatively, I think he could be an Alan Rickman type (appreciated for a diversity of roles, but never rising above the status of beloved character actor).
Emma Watson. Durable. She's in dangerous territory right now: being an It Girl and known as a fashionista. That always limits many an actresses shelf life. I think she'll escape it because she's getting a college degree. If there is anything Hollywood loves, it's stars with college educations. Think Jodie Foster, Natalie Portman, James Franco. It will add another 10-15 years to her career, and by then she'll have figured out her niche and probably have done some stage work and more indie movies.. She keeps doing ingenue roles though--which she needs to stop doing.
Rupert Grint. He'll surprise us. Of the golden trio, he's the comic, quirky one. Which is reflected in the non-Potter films he has done. Britain cherishes its character actors, so I think his career is safe. He can do drama as well as comedy, but he's not been given an opportunity to really prove his chops. I think because he's currently underestimated, he'll be forgotten, but then 10-15 years from now, shock us all in a way neither Emma nor Daniel ever will.
Tom Felton. I hate to say this, but I think he's most in danger of fading first. So far, his career has consisted of: the cute son, horror, and the bad boy. It's basically the typical "paying your dues" arc for a child/teen actor--and many a teen actor career hasn't survived this arc. While his charm takes him far in interviews, it won't get him anywhere if he can't prove he can act. He needs to find a meaty, challenging non-teen arc type role. And he needs to start picking roles that don't scream "typical teenage boy" or "bad boy archetype."
Matthew Lewis. He too can do comedy and drama, but he's quite modest even for a British actor. Given how egotistical the film industry can be, I hope his modesty doesn't cost him jobs, but I think he'll have to fight harder to get roles. I think he'll keep dabbling on both stage and screen, but likely his home will be British TV, and he'll be more of a character actor.
Jessie Cave. Poor girl already has been rejected by Hollywood! Despite that setback, I think she still has a solid career ahead of her. Proof of her durability comes from her own website where she produces shorts as well as her own fashion line. Of all the supporting cast, I think she'll be the first to break out of the ghetto. Plus, she can do comedy and drama. I think she probably has awards in her future.
Evanna Lynch. I'm on the fence. She's enchanting, and she's such a Potterhead, but that doesn't translate into career. Of course, she'll probably keep appearing in Jessie's shorts but I think she'll probably end up...not acting. I think she'll probably keep being involved in Potterverse, but I think she'll eventually fade. (My opinion is based on the fact that over a year after HP has ended, she has no film projects lined up. She mentioned school, but even if school keeps her busy, she could still do bit parts in films or TV. And she hasn't.)
Alfie Enoch. Well, he's from an acting family and he's going to Oxford, so that pedigree alone will help him go far. I think he'll keep acting and he'll has a good career ahead of him.
Devon Murray. He's an equestrian! Plus, he's passionate about his farm! So he's fine if he decides to quit acting. But I think he'll do indie films and more bit parts.
