This may be true. Tatum and Brown both have huge upside, but that doesn't mean they have the IQ yet to make the right basketball play each possession.
This is what Hayward and Horford both bring to the table. They will not only exemplify making the right play, but their play will enable Brown and Tatum to take good shots and be highly efficient.
When I say that I think Hayward might come off the bench next year, it's not because he isn't invaluable, or because I think he is worse than Brown and Tatum. I think he is better than they are now, but I think he has the empty ego and high IQ necessary to fit in around everyone else, making high IQ basketball plays, and raising the entire quality of the team.
I could easily see Hayward and Horford's numbers lower than Brown, Tatum, and Irving next year. But every person on the team has too make everyone else better. Tatum and Brown can't do that yet (and Irving is still learning how to do that), but Horford and Hayward can.