As someone who grew up reading Disney comics from both Europe and America, I must wonder, why is it that some writers replace Flintheart Glomgold with John D. Rockerduck? And mind you, I'm not asking why Rockerduck is used, but I'm asking why writers who use him can't use Glomgold as well? Because I've found that Glomgold and Rockerduck are, in fact, very different characters. Whereas Glomgold is essentially Scrooge as Scrooge would have turned out if he hadn't made his money square, Rockerduck is a more modern businessman. Also, stories tend to portray Glomgold as more of an out-and-out villain and Rockerduck as more of a bussinessman who isn't that bad a guy outside of his grudge against Scrooge. So I must ask myself, why does no one use both characters?
Rockerduck lives in Duckburg, Glomgold in South Africa, so that's one explanation. The three sometimes used to compete when three millionaires were needed.
Both Glomgold and Rockerduck were created by Carl Barks, though Barks only used Rockerduck in one story (and in a fairly minor role, at that). For some reason, Italian comic artists took the character and made him a major player in their version of the Disney Ducks Comic Universe, more or less giving him the "second richest" role that Barks had given Glomgold. Which may be one reason why the two characters aren't used in the same stories: two "second richest ducks" might just be confusing, and the rivalry might not seem as intense and personal. That said, it would have been interesting to see how Glomgold and Rockerduck might relate to one another; I can't imagine they'd get along.
One Italian story actually showed them cooperating to ruin Scrooge. After their apparent triumph, however, Rockerduck being a mostly decent person and Glomgold being a Corrupt Corporate Executive caused them to stop thrusting each other and expect to be backstabbed (and that's what allows Scrooge to trick them into selling him back his empire: they both tried to backstab the other first).
When you think about it, Glomgold's a white South African (I assume he's white, though of course the ethnicity of the ducks is kinda ambiguous) who's made his fortune by dirty means, so maybe the Italian duck artists thought such a character would have too many Unfortunate Implications to be in a kiddie comic. (The left wing in Italy is much stronger than in the US, so presumably the anti-apartheid movement was stronger there too.) As an American billionaire (who, as mentioned above, seems to have gathered his fortune by more lawful means than Glomgold), Rockerduck might've been considered a "safer" antagonist to Scrooge.
He's Scottish just like Scrooge
Glomgold? Only in the DuckTales universe. in the comics universe, his nationality his less certain, though according to Don Rosa, he's a boer. Rockerduck, by contrast, is very clearly an all-American businessman born into money.
The name "Glomgold" definitely suggests Dutch/Boer ancestry, as does the fact that he lives in South Africa. DuckTales changed this, either for simplicity's sake, or because it was a bit... problematic to incorporate an Afrikaner into a children's cartoon in the late 1980s without wading into the hot-button issue of apartheid.
It's only referenced a couple of times, but why does Scrooge maintain a private zoo? We hardly ever see him there unless it involves his unicorn, and it's gotta be a huge drain of money to maintain it and not open it up to the public. I know he's a nature-lover in the vein of Roosevelt but it doesn't gel with the rest of his character.
[[WMG Breeding program for the animals he discovers, so he can sell them as exotic pets to millionaires.]]
In the Dutch languague version of the catching of the unicorn, that zoo is open to the public, but the entrance fees are sky-high, further private zoos are status symbols,for the rich and mighty, so he might need the zoo to compete with Glomgold and/or Rockerduck.
He might also view the zoo as the "living creatures department" of his treasure collection. After all, a such extremely rare animals like the unicorn are highly valuable, just like the Golden Fleece or the crown of Genghis Khan. (Although granted, the original Barks comic suggests that Scrooge follows a quite completist approach concerning his zoo, which means it contains not only rare, but also very common animals.)