I think Zirkzee has the tools to become a deadly out and out number 9, but he's not that kind of player right now, and even if he goes on and develops into something like that, it won't be for a few years yet, but that's fine, because he was brought in for being a facilitator type "9.5", not because we want great end product from him. You can basically consider it a bonus that he might develop into something great later in his career, but he's a really useful player already, and we already have a great out and out number 9 with potentially elite level end product in Hojlund. I think people don't realize two thing: How good Hojlund is, and how small the current talent pool for potential 30 goal a season strikers are right now. Hojlund is one of the very few under 21 players in the world who fit that criteria: it's probably him, Sesko, Endrick, Omorodion and Evan Ferguson. And Endrick and Hojlund are already at "final destination" level clubs. We should be happy with our current options IMO, as both of them will contribute a lot this season.
Overall, Zirkzee has added good tactical flexibility to the squad and potentially good versatility as well (although I don't see either Bruno or a potentially in-form Hojlund getting dropped for him, so his versatility will be limited). Another poster compared him to Matheus Cunha and I think that's the best comparison I've seen so far and the one I like the most, but I think there are also some similarities to Firmino, Havertz, or even Kane.