He is the final piece, to win the CL and be overkill in the Prem
He's a replacement for Aguero who used to score loads of goals for them.
City have got other issues to sort out they've lost their squad depth with a few departures if they sort all that out who knows what happens.
City already bang in 90+ points a season most seasons I don't think Haaland really changes that to much and in Europe we've all seen it can come down to luck in that competition the best team doesn't always win it. Eventually it will be City's turn to get over the line but I honestly don't think Haaland guarantees that.
City's problem that I see right now is they don't really have a left back, they're little bit light in midfield and in wide areas for numbers. If on top of Haaland they had another 2 or 3 quality players this summer then yeah they'll be flying again.
The challenge for Utd is to build a side that is also capable of delivering 90+ points a seasons like Klopp has done at Liverpool as that is the standard now.