Not sure it'll justify the salary, but he's been consistently improving every year since he got to the league. Yes, everyone sees the gaps, but even with them he is a 2nd team all nba and has yet to reach his peak (imho). It's a massive contract but the front office believes in him, and that's something to keep in mind.
Based on them being as successful as almost any young duo ever has been as the 1-2 stars on a team?
Look when other all time greats won their first title. The difference between Tatum/Brown and others at the same age isn't not winning a title. It's how consistently they've been in the mix.