So who?
Alvarez was the biggest talent in Argentina that every big club knew about and known throughout the world. We just didn't get him because we don't make good deals. You make it sound like City took a random gamble on him and he turned out to be good.
If there was a player like him now, we'd be all over it, but there isn't.
Just because there is a striker that scores goals, doesn't mean he's a great striker that will fit our system. In that case, we can just take our pick. Elye Wahi, Folarin Balogun, Jonathan David, Lacazette, Openda, Ben Yedder, Moffi, Gouiri, Habib Diallo, Lautaro Martinez, Boulaye Dia and more.
Plenty of average strikers score goals, which is why, when scouting a young striker, you don't necessarily look at the goals, but the combination of abilities and characteristics that player has.
For a 20 year old, Højlund has all of it that can potentially be nurtured and making him an elite striker. He has the potential to explode if he can improve these abilities. Most of the players that I listed above are lacking in one aspect or another (or more), but Højlund has it all. He has strength, pace, movement, the natural instincts that Cavani had, acceleration, hold-up play, link-up play, intelligence and more. This is what makes him the ideal target, and not Boulaye Dia because he scored 15 goals, and this is probably what Ten Hag sees in him too.
There is no guarantee that he will knit all this together, and that's okay because that would make him world class, but Ten Hag sees the potential in him and believes he can help him be a great striker.
Another thing people forget when thinking about signings is the mentality and attitude aspect Ten Hag is looking for in his players. Which of his transfers have looked weak mentally, bad attitude or not willing to put in the work? Absolutely none. They're all motivated, hard working, fierce and strong mentally. It would appear Højlund scores high here too.