Haaland will probably move on to Real within 3 years. Alvarez will be 26 then, coming into his prime and then he can get his 30+ games a season (if he hasn't already broken into the xi, since he's already moved into midfield to play alongside Haaland for some games).
On the point about us not being one of the best teams in the world, fair enough. That being said I think this Utd team with Kane and a top midfielder would be among the best personally even if they might still be behind a couple teams like City and Real. Bruno, Casemiro, Shaw, Martinez, Varane, Rashford and Kane are all among the best in their positions. Even looking at the CL semi finals, I'd take these United players over quite a few of Inter, AC Milan, Real and City's players in specific positions (varies depending on each team, City's squad is the best but I'd still take Shaw over any of City's left backs, I'd take Bruno over Valverde, Casemiro over any of Real's dms, and any of the 7 over any of AC Milan or Inter's players tbh). There are some other areas that need to be improved like rb, gk, rw but the team is stronger than many give it credit for.
Already mentioned I think Hojlund and Kane could play together