By now everyone knows Oswald, so Julius is the GOAT of Disney trivia. At this point he's basically a footnote, but I can respect the legacy. So what if he's a Felix ripoff?
Walt Disney's distributor was the reason he turned Julius into Great Value Felix. They want to ride Felix's popularity and Disney was in no position to negotiate. But it worked! It turns out you can do a lot with a wacky cartoon cat than a live-action little girl. Julius gave Walt the push he needed to ditch live action and embrace full animated cartoons.
So if Julius was legit influential, why did he disappear? Well he was still a Felix ripoff, and Walt always hated being unoriginal, especially when he was forced to do it. Plus Oswald was better than Julius in every way tbh