Not just the media.
Fans too.
I think the following factors were an important reason why we/they miscalculated:
1. The degree to which Kyrie hurt team chemistry, and growth of young players;
2. The impact of losing Horford was exaggerated; his replacement cost through committee has been fine;
3. The early chemistry that would be facilitated by Team USA bonding of 3 Celtics with new Celtic Walker;
4. How good normal Gordon Hayward is, and that he would become normal again;
5. Brown and Tatum would be able to jump back onto the growth curve, after treading water last season; see 1 above;
6. Roster consolidation (i.e. losing Rozier and Morris) enables Brad to jack up minutes of his top 5 players, keeps them happier (we now have 4 potential 20 PPG players, and Smart, who could care less what his PPG are);
7. Media has been caught up in big two musical chairs, and loves to speculate how the rosters with multiple stars (LA, LA, Philly, HOU) will fare;
8. The Western Conference bias; it's better 1-8, but not 1-5, IMO;
9. How good a coach Brad Stevens really is, when ALL THE PLAYERS buy in, and implement his system; this team is beautiful to watch (first and last games excepted);
10. How much of a leader Kemba Walker is, and how perfectly he fits in with his teammates and coaching staff.