Mpappe would probably cost more then what we could get for Suarez.
He'd probably cost twice as much as we could get for Suarez if not more.
Even if we forget about Messi-Suarez relationship that makes it impossible for us to sell Suarez I don't know why we would try to replace a proven worldclass striker who has a great chemistry with Messi and rest of the team with talented kid who we don't know how he'd really do here. I like Mbappe and don't doubt about his talent but he'll be overpaid by whichever club will sign him. It's also very possible that he won't meet high expectations at his new club because of added extra pressure on him due to very high transfer fee he'll command. Most people forget that. He wouldn't be the first nor the last highly touted youngster that has moved for big money and then never reach his full potential.
Yes, he might turn out to be next Henry but Titi also failed at first when he moved from Monaco to Juve. And he was 4 years older at that time than Mbappe is today and his transfer fee was also much lower. Just for example: Juve paid £10.5M for Henry in 1999, the same year that Vieri was sold from Lazio to Inter for £32M. Figo's transfer fee was £37M and Zidane's £47M over the next two years. I know transfer market has again gone crazy over the last few years (the same as it was at start of 21 century only that financial power has flipped from Serie A to EPL teams) and nowadays clubs are paying like 70% of transfer fees for potential of the players and not for what they've already done so far in their careers but I just don't think Mbappe is close to being a top 3/5 players in the world right now which is most likely the price range he'll fall into. I think he's closer to what Henry was in 1999 than what Figo or Zidane were. You pay 80M or more for players that have played on top level for many years and not just for someone's potential. Let's be fair, how many people that are ready to pay crazy money for him now did even know Mbappe before this season? Probably not many.
I get why people like him and think he could be the next big thing but he could just as easily disappoint at his new club. That's why I think it's just not worth the risk, especially at the expense of Suarez who we already know fits into our team perfectly.