Who's better? Find the money from where?! Have you not been following the news around the club at all? Sacrifice which position? Because most of them need at least a partial upgrade, but up front we only gave Højlund.
There's obviously a reason they want to sign him, which has been discussed in this thread numerous times already, although I imagine the fact he has a reasonable release clause has pushed him ahead of other potential targets. We're not going to pay silly money for someone like Osimhen, and while I'm sure our scouts have a list of young strikers, it's unlikely any of them would be ready to play straight away, unlike Zirkzee.
We finished 8th last season, can't offer CL football this one, can't guarantee regular CL football at all right now, so it's unlikely any half decent striker will want to sit on our bench.
Zirkzee is basically a replacement for Martial but might actually stay fit for half a season. He probably won't pull up any trees, but offers something different, won't have massive wages, and leaves plenty left in the transfer budget without immediately needing to sell. Which is important, as nobody looks like they're in a hurry to sign Casemiro, Sancho, or Greenwood.
If/when we want to improve on him in a few seasons, selling him on shouldn't be a massive burden, like it has been with some players.