Why are we talking about kane vs haaland in terms of "downgrade"? Kane is a better player than haaland right now, I hope that's clear to everyone.
Really hard to compare since Haaland hasn't played in the PL yet. We dont know if he would score 15 or 30 goals next season if we or Chelsea signed him. The only competition we can compare them in is the Champions League, where Kane has 19 goals in 2088 minutes played (109.9 mins/goal) and Haaland has 20 goals in 1259 minutes played (62,9 mins/goal). Haalands numbers might be unsustainable, and its not only about goals, but its harsh to say Kane without doubt is the better player.
Of course there is a risk that Haaland could be somewhat of a flop, taking time to settle in, or maybe never really get his mojo going. But its not a guarantee Kane will be a success here either, Fernando Torres and Alexis Sanchez are examples that show it does not always work out just because the player knows the league.
I would say they are at a similar level in terms of current ability, but with Haaland having the potential to be the best striker in the world for the next 10-15 years. When you add in factors like Haaland been a fan of the club since his father played here and Kane for me will always be synonymous with Spurs the choice is simple if both are available. For me, Aguero is a better striker than Kane. Unfortunately Sergio couldn't be trusted because of his injuries but I think Kane is a downgrade in terms of ability. Kane is also likely to not improve much, while Haaland has the potential to actually be better than Kun. All things considered, if Chelsea get Haaland and we get Kane, it would feel like we got the second best option.
With that said, there have been countless of occasions where what seems to be the second best option in advance, turns out to be the best one. As long as we get one of Haaland, Kane, Mbappe or Messi I'm going to be happy, anything less would mean letting Aguero go was a mistake in my opinion.