Make no mistake - a healthy Beal is better than a healthy Hayward. I think (hope im wrong) Hayward will join the ranks of guys like Jamal Magloire and Tom Gugliotta in a few years as a guy people say - "he made an all star team once?!" A lot of Celtics are disillusioned about the type of player Hayward was even before we got him - a career 15/4 guy who isn't really elite at anything on the court.
I'm not going to argue over whether healthy Beal is better than healthy Hayward, but I think you're severely underrating Hayward here.
Last healthy season for Hayward:
21.9 pts, 5.4 reb, 3.5 ast, 1.0 stl
53.6% EFG ... 39.8% 3P (5.1 attempts) ... 84.4% FT on 5.9 attempts
.199 WS per game, 10.4 WS total
That wasn't a total fluke season, either. Hayward was really, really good for three straight years before signing with Boston and getting hurt.
Hayward also played 72, 66, 72, 77, 76, 80, 73 games to start his career before missing all of last season (except for a few minutes). Availability is a valuable trait, as well.