You didn't understand what I said. I said to play as a #9 you'd need to be extremely fast. Or strong enough. As we agree he's not strong enough yet so he'd need to be extremely fast. Problem here is he has some decent pace but definitely not extremely fast. He's simply not ready yet for the PL atm imo.
And under ETH we have moved from counter attacking football to possession based football. In this new system for a #9 pace is not really important anymore but he has to be at least able to hold the ball under pressure, can pass and bring the other forwards into the play and outmuscle the defenders to get into the position to score. And of course know how to finish. That's why despite having almost no pace Kane is perfect for ETH.
It's not easy as you say put him in the gym and he'd get strong as an ox in no time. And you gotta balance your weight, your agility and your pace. It's very rare to find a striker who is tall, big, strong and also fast. That's why despite having a pretty shit first touch Haaland is still a monster.
Hojlund might get bigger in the gym but not surely stronger. For example Lukaku looked like a beast when he was 15 yo and everyone thought he'd become the next Drogba. I'm sure he hit the gym as well but if you watched him when he was with us you'd know despite looking huge he got bullied all the time by the PL defenders and always lost the ball being challenged by them. And Hojlund so far has proved his hold up is ok in the Serie A only. Where Lukaku could pretty much bully the defenders there. The PL is simply much more physical and difficult.
I really hope developing a top #9 would be simple and easy as you said but unfortunately it's not.