There is no such thing as almost certain improvements. Brown and Tatum could easily stagnate...again(because their "almost certain improvements" didn't happen last year) and Hayward at the end of 2018-19 could be the guy that is here in 2019-20.
A wise caution.
And yet, I believe in Brown and Tatum, don't think that it's probable that they won't progress, and am especially encouraged by what I've seen this summer.
About Hayward at the end of 18-19: I thought he finished the season well, the last three months especially.
Add those uncertainties to the inarguable step back in talent and ability up front from Al, Baynes and Morris to Kanter, Poirier and an out of position Hayward or Tatum and that is where people are getting the idea things aren't as great as some may think this year
Again, perfectly rational, more than a fair point, and even apart from whether or not Kemba is a downgrade from Kyrie (or Kyrie plus Rozier). As you say, this is inarguable.
The loss of Al Horford is hard to calculate, in particular: it's not obvious to the casual fan how important he was in leading and directing the team defense, for example (another good reason for Marcus Smart to be in the starting 5, btw - though it's true that bigs have a better view).
Baynes and Morris are lesser losses (though they point to the fact that Boston has lost size generally, or more exactly veteran size).
And yet...
Perhaps it's the August optimism that all teams and their fans always have, but despite your "inarguable" points (and I'm not arguing- you're right!), I see "a thousand flowers blooming" on Boston's roster, space (and shots) opening up for The Two Jays; Hayward returning from perpetual rehab wilderness, freeing him to do what he does; Hayward, Smart, and Kemba bringing leadership and teamwork; Kemba not having to carry the team, freeing him to do what HE does; rookies pushing, vets pulling; the emergence of RW3 into an impact player...
I'm predicting a big step forward for the young Celtics, and I'm taking the over.
51.