Tatum and Brown are definitely redundant skill-wise. It doesn't mean you can't win with them, but they need to be surrounded by very complementary players. Some stars are more versatile in adapting to what's around them. I think the two Js will continue to improve their vision with more experience and trust in better teammates, but I don't think they will ever have the Doncic or LeBron gene where they're essentially point forwards. It's not a problem now, but maybe one going forward if it's just too difficult to fit all the perfect pieces around them.
The other thing is personality. It's not just maturity, because some guys come in the league and instantly are leaders and dominant personalities. Tatum isn't that guy. He's more like Kawhi in that respect, a great player that isn't very vocal and doesn't necessarily facilitate for others much on offense. On the Spurs and Raptors he didn't need to do anything else because they were already established in the leadership department. Our team doesn't have that because we didn't keep any of our star veterans, for various reasons.
I think Brown definitely has leadership potential, but he seems uncertain and insecure sometimes. He's also soft spoken and kind of introspective, which is fine but again, not really helping fill what they appear to be lacking right now.
So, what do they really need? Well, let's see. They need a PG to help run the offense whose game is more about facilitation than on-ball scoring. He has to be a good outside shooter on the catch and, if we're trying to be a contender, is at least solid defensively with OK size. They need a PF that can switch on D and guard most positions effectively, including the 5 sometimes when they go small. He also needs to at least be OK at 3 to keep D's honest. Preferably, he also is a solid passer though if they keep Rob Williams then that's less of a requirement. Preferably, both guys are providing leadership and toughness and aren't liabilities in the size department.
I know what you're saying, how are they going to get these seemingly perfect guys? Well, the first thing is they need to actually be committed to paying the tax. The second thing is, it would really help if they actually knocked one of their late picks out of the park (think Draymond, Jokic, J. Butler, Tony Parker, even Giannis) instead of just getting an OK but low-ceiling player with critical flaws. This means they need to stop taking guys that are way too short/small to be anything more than bench filler unless there truly isn't anything better available. In a similar vein, they need to find some gem or "second draft" guy in free agency domestic and/or Europe that turns out to be better than your average signing. Even if they can't afford to keep everyone long term, that's often the only way you can sneak excess talent onto your team when hard-capped. The third thing is they obviously need to eventually turn Kemba's cap space into one great player, or 2 maybe even 3 productive vets.
It would be a lot easier if there actually were stars available that fit better with the Js, but it doesn't appear to be any really obvious targets right now. Beal is just another scorer who is undersized but not really a PG. If Portland (Boston West) blows it up, their stars are also similarly on the ball scorers. Chris Paul would have been great 1-2 years ago instead of Kemba, but he's older now and the C's don't have the means to sign him currently anyway. Lowry is the same thing, too old and hard to fit financially. Jrue Holiday was a fit even if he's not really a leader, but obviously Kemba's bad contract made that an impossible pursuit and he's no longer available anyway. I'd really like them to kick the tires on Lonzo Ball but I'm not holding my breath because they don't have the assets/flexibility to go for him realistically. Mike Conley is a good target, but again do you have the maneuverability to get him?
As far as PF goes, Barnes is a nice fit, but I don't really think he's known for being tough or a leader, so you still need that from somewhere. It also would have been amazing if they had gotten Julius Randle a year or two ago but that ship has sailed, obviously. John Collins is talented, but even if we could acquire him it's hard to see a team of him and the two Js on three big money deals being enough to actually win a title. Myles Turner is still likely available but let's be honest, he's really more of a Rob replacement, not a PF.
The more you really look at it the more you understand why almost no commentator or writer has a very positive outlook on the team's near future. They need to develop what promising young guys they have to improve their team but also to increase their trade value. They need to wait until Kemba's deal is more tradable. And they need to keep Jaylen for now because he's still improving, on a cheap deal, and I don't see any other young star available that would be a considerably better basketball fit with Tatum right now. Also, unlike Tatum he actually has shown signs of being a leader which is more than I can say about some of these guys who put up numbers but are suspiciously always rumored to be available.
Finally, let's be totally realistic here. Let's say they strongly believe internally Brown and Tatum are probably a little bit redundant. We know they prefer to emulate the Spurs and Jazz with great passing, court vision, defense, consistency, and selflessness. Does that sound like Brown or Tatum? Not really! But it doesn't mean you just get rid of them even if you think they might have careers that are more similar to Carmelo or Iverson or Pierce? Meaning, they were scoring stars that needed to be the second best player on the team, not the best. Does it mean that maybe they are more like the franchises that only won one title as opposed to 3-4 (e.g. the Mavs with Nowitzki, the 08 Celtics, the Pistons)? Maybe, but if you don't actually have a realistic way of getting those super duper stars, just being competitive is still incredibly valuable. Maybe you win one title in a weird year. 29 teams don't win the title each year. Just winning one is incredibly difficult.