I understand the series' justification for removing Donald from the picture except for occasional guest appearances, but I still think this was a dubious decision, as it substantially unbalances the show. It's not just out of random chance or inertia that Donald appears in every single one of Barks' Scrooge adventures (he did a few Donald-less Scrooge shorts, but aside from the one-pagers, that's all), as well as virtually every one of anyone's. Donald serves as a counterbalance to Scrooge, at various times raising objections to his uncle's pie-in-the-sky plans, complaining about his stinginess, plotting to slack off instead of doing his work, and, from time to time, rising to the occasion and getting things done, coerced or not. This is very nuanced stuff, and even a talented cartoonist can screw it up (witness the sadistic abuse that Don Rosa inflicted on him a little too often), but done right, all this comes together to make him into, essentially, the stories' viewpoint character--the Everyduck with whom we identify. Everyone likes Scrooge, but his concerns are far removed from those of readers (there's a Barks quote floating around out there acknowledging this), and he obviously is very specialized--all of which prevents him from really being universal. Likewise HDL: they have a better claim than Scrooge, but the fact remains, even at their most mature, they're kids with kids' concerns. Donald's the one who serves to keep the whole affair grounded; indeed, I would go so far as to argue that, aimed at children though they are, these comics aren't likely to be fully appreciated by kids themselves*, who aren't really in a position to be able to really understand Donald. I certainly liked him well enough as a young'un, but my real identification was, naturally enough, with the kids, which I think is slightly missing the point, awesome as they are.
*That's right: I appreciate duck comics on a much deeper level than you.
Of course, in Ducktales, Launchpad takes the place of Donald--indeed, there are a few comics which it's clear were hastily retooled to substitute Donald for Launchpad when Ducktales comics ceased to be a going affair. I like Launchpad; he can be a great character when well-written, and he's generally acquitted himself well in his occasional starring turn. But he's not a substitute for Donald: he's too broad and cartoonish, with his ostentatious dumbness and pride at his constant crashing, to play the same role.
All of which is only to make the point that Fenton is a great addition to the show because he plays that everyduck role, and plays it very well. Not that he and Donald are interchangeable, of course, and stories that focus on him are not stories that could very well have been written with Donald in mind. But he's a generally sympathetic character with strengths and weaknesses and relatable, real-world concerns (secret identity notwithstanding), and he really provides the show with an element that I didn't even realize it was missing until I saw it.