2. Pro: Keeps Lauri Markkanen around long-term
Obviously one of the pros is that by re-signing him to a new deal now, you keep him around for another four or five years at least. He'll likely want to win and win soon, so you just can't keep tanking at the end of the season, but the idea is that if you want to re-sign him, you aren't looking to tank. Markkanen only gets a new deal if the Jazz really want to keep him. Right?
2. Con: The team may re-sign him to trade him
Wrong, as a fan, there is a con to him getting a new deal; it's not a sign of a team ready to win now, but it's a sign that a team is ready to trade Markkanen and they're giving him a new deal to help make him attractive to other teams. Trading Markkanen would essentially mean years of bad basketball to come. While some would say that re-signing him to just trade him for assets is a stroke of brilliance, I for one want the Jazz to win. Watching losing basketball isn't fun and if something isn't fun, why bother doing it? So trading him would likely devastate the fandom on an untold level.