Look, I'll be the first to admit it's entirely possible the team could win MORE games next year if they head into next year with substantially the same roster.
It's not just entirely possible, it's more likely than not.
You ask things like why people are "certain" that we'll be better. Sure, I said definitively that we "won't" be worse, just like many of those that agree with your point of view say definitively that we "won't" get Love. Everyone agrees there is an existent albeit slim chance of landing Love, but it is "slim" that is the key word which makes such absoluteness excusable.
In other words, there is a chance that 5-6 of our best young guys look worse than they did last year, but the chances of such an outcome don't seem significant enough to me to even inject it as an alarming possibility into the conversation.
Knowing the NBA as you obviously do, I'm just not sure why you bristle so aggressively at the idea that the Celtics, without major changes to upgrade the roster, could win fewer games next year.
Unless you really think that dumping Rondo and Green and adding Isaiah Thomas and Jae Crowder to the roster was really enough to make this a sure-thing 45-50 win roster moving forward, as opposed to the 25-30 win underachiever it was with those guys in featured roles.
You've argued that it's irrational to believe that more or less the same group of players, minus a few veterans, could be a worse team (record-wise) next year than this past season.
While I can see that point of view -- you have a young team, young players have a tendency to get better over time, and the team posted a much better record in the second half of this season compared to the first half -- I don't think it jives with the way things tend to go in the NBA.
If you believe, as I do, that much of the success of the Celtics over the second half of this season was due to:
(1) opponents' lack of familiarity with this group,
(2) opponents' lack of effort or interest in winning games compared to the Celtics, looking to make a playoff push, and
(3) the fact that many of the players on the Celtics were eager to prove their worth, because they were in a contract year or because they had just gotten dumped by the team that originally signed them
then it doesn't seem so unreasonable to suggest that if the team lets go of Bass, Jerebko, and Crowder and replaces them with inexperienced, unproven, underdeveloped young players, that the team might be worse.
This attitude that the young Celtics will almost certainly improve and build on their run to the seventh seed tends to disregard the reality that almost every other team in the conference intends, and even expects, to improve next season, as well. All it takes is for a few veteran teams that fell off this past season -- e.g. Heat, Hornets, and Pacers -- to return to form and suddenly the Celts are on the outside looking in, again.
There are any number of factors that can go into a team succeeding or failing in the regular season. We could simulate this past season with the roster the Celtics had in the playoffs and the win totals could vary wildly, from 25 to 50. The young guys on this team could improve next year, Brad Stevens could continue to do a great job as a coach, and this team could play at or near the same level as they did in March and April of this season, and the Celts could still win up with 5-10 fewer wins than last year.
Sorry for the aggression. Your camp (but not you) of thought on this issue tends to adopt a domineering intellectualism about themselves when explaining their point, so I feel I have to dish some of that condescension back the other way
The way I understood the argument was, if we brought back largely the same team (so Jerebko leaving, Datome leaving, and replacing Bass' minutes with Sullynyk), how would they fare? You bring up a number of points that support that they would be worse. I agree with them. Some guys won't improve as much as we like. Some guys - like Evan Turner - could be even worse. We'll play, on average, better teams than we did at the end of the season. More crucial to your argument is the reality that teams will come to play us every night, instead of just taking us as a joke.
All of that being said, some of our guys
will get better. Again, I can't think of a team in the past with the makeup we have now (5-6 guys 25 and under, 7-8 guys 28 and under) that saw all of their major young pieces stagnate or regress. If there is/has been, that team is the exception, not the rule. So ultimately, even if the negatives end up outweighing the positives (more players regress than improve, other teams' preparation for a legit Celtics squad trumps our players' increased confidence that they can win, etc.), I do not see it having the
drastic impact you are claiming it could. We played .645 ball after the ASB (and won 4 of 5 heading into the Break). I've conceded that this currently constructed team could fall to .500 ball. You're going as far to say they might only be able to scrap out 30-35 wins, which would be as much as a 20-25% drop in Win Percentage. That's too extreme for me.