I like Malen but don't believe he's a "killer" we need. He's clearly much more affordable than Haaland though and if really available for around 25m probably worth a risk. He's younger and with more upside than Depay and also quite versatile so if Koeman stays beyond this season (which he probably will if Laporta is elected) Malen could do well in his system with 4 players upfront constantly moving around and interchanging their positions.
Personally I'd let Messi go and sell Coutinho (to clear debt and possibly reinvest some money in other necessary signings, mainly a CB) and we can go through next season with Fati, Griezmann, Malen, Braithwaite, Dembele, Trincao, Pedri and Puig for attacking 4 positions. Alternatively MB (as much as I like him) could be sold if we get a good offer as I think Malen could provide things he does and we could replace him with a cheap veteran like Giroud who would be a good "option B" against parked buses.
Ideally Haaland will stay in Dortmund until 2022 and we could offload Griezmann then and get him if our finances would improve enough but if not we'd have another year to scout other alternatives. But who knows, maybe Malen and Fati could turn into 25-30 goals players and then we wouldn't so desperately need someone who would score 40 goals every season. Sure, would be great to have that kind of a player but they are rare and usually very expensive so having multiple players who can score a bit less but still well enough is the next best option.
As long as our defense would improve as well and I've said it before I believe in post-Messi era we should first build a defensively more stable team until we'll be searching for a new offensive identity which could take some time after being so reliable on one player for so long. Not saying we should turn into the next Atletico but would be great if we would also learn how to win 1-0 sometimes without struggling too much when forwards won't have a good day and score 3 or 4 goals.