People are really underrating Kemba. We have him for 3 more seasons - longer than Smart - and he is easily a better player than Smart. We'll have to see how his knee situation goes, but it sounds like he can still basically be himself, he will just have to be managed correctly.
We are going to miss Hayward, but I don't think we need a Hayward caliber player to make up for his absence. Guys like Winslow, Gordon, Barnes, even Joe Ingles are more than enough to pick up the slack of Hayward who wanted a bigger role. Who knows? Maybe somebody like Grant Williams is enough or Nesmith/Romeo prove to be worth their lottery pedigree.
I would say Tatum and Jaylen are good enough to be your top 2, but they aren't Lebron/AD level - you need a strong 3rd scoring option (currently Kemba) and very good two-way players to fill out the roster.