This team won't be a real contender until Hayward is the best player on the team. Irving just isn't good enough to be the main guy on a champion.
The best guy on a champion in a league with the Warriors in it, or a league with normal talent distribution?
Right now, nobody is good enough to be the best player on a champion unless they're on the Warriors.
In a normal league, I think Kyrie is absolutely good enough. The combination of elite defense and an elite individual scorer is a good recipe for a title, if some things break your way and you have home court advantage.
That isn't true. Plenty of players are good enough to be the best player on a champion, they just may not have an overall team that can compete with the Warriors (I mean Harden's Rockets were basically a Paul injury away from beating them last year). I'd put the players that are good enough (i.e. Gold Medal Superstars) as Lebron, Durant, Curry, Davis, Giannis, Leonard, Harden, and Westbrook. I think Embiid could be there in a season or two and guys like Dirk, Wade, and Paul were, but aren't at that level any more (obviously Paul is closer to that level right now than the others). Guys like Irving and Hayward are in the next tier down (along with players like Lillard, George, Butler, Jokic, Paul, Embiid, etc.) such that in a rare season and when coupled with multiple similarly skilled players a team might be able to win a title, but that team likely isn't going to be in the running year after year. The Pistons of 2004-05 are an example of that type of team. A nice 2 season run, but needed some crazy luck to actually break through and win the title.
To be a real contender this year, Boston absolutely needs Hayward to get back into form and needs Tatum to continue to develop. The team would then have 3 to 4 silver level players (Hayward, Irving, Tatum, Horford) and a bronze or 2 (Brown, Smart or Rozier), such that the right luck and they could beat a team or two with a gold medal superstar and win the title (again much like the Pistons did beating the Lakers). Though as you say the Warriors aren't like most other contending teams, which poses a real difficulty not usually around (though an injury to Curry or Durant and that Warriors team becomes beatable).
Only if Tatum reaches max potential and ascends into the gold medal category (or Boston acquires that player, like say Davis) will Boston be a consistent and yearly contender for years on end.