I think you really shouldn't be using those series as reference points as there aren't many series that reach that amount of chapters. These days it has become much more common te create series of a moderate lifespan rather than series that last decades. At this point some kind of a generation switch is occurring in the manga industry. Series like Boku no Hero Academia are made by people who grew with series like Naruto and One Piece and when such a with occurs, it's accompanied by a new perception. By now the amount of series has exploded, so having many decade-running series isn't really cost efficient. They don't even plan that far ahead, normally they just make an estimation of how long or how many chapters a series should consist of. When it appears then that it's beneficial to keep it longer, they will. OP and FT never intended to be that long, but they kept it going because they're so popular.
If Black Clover wants to reach something like 800 chapters, it will have to put in a lot of effort. It's an enjoyable series that can easily gain popularity, but that's something different than OP. OP is pretty much this generation's DB.
