I've already said that I think 49.5 wins sounds about right.
With that said, I think focusing on the notion of whether last year's team had the "talent of a 59 win team" or the "talent of a 52 win team" is too general and impossible to really argue. Talent is somewhat subjective.
I don't think losing Kyrie means very much. He was a great scorer but he was a toxic priss in the locker room and promoted bad habits on the floor. Also, and more importantly, the team replaced him with a very similar player.
For similar reasons I don't really think it makes a difference that they lost Morris or Baynes. Both are very replaceable.
Losing Horford is a huge deal. The team's system on both ends of the floor hinged on Horford's unusual, extremely versatile skillset. You can't just talk about Horford's talent on its own. Horford isn't a superstar, in fact isn't even a clear cut All-Star anymore. But his skillset is very hard to replace and the Celts most certainly did not replace it this off-season.
Even so, I do think the team underachieved last year, both in terms of the actual wins they produced despite their point differential (as saltlover pointed out) and in terms of chemistry ruining their focus and promoting bad habits that contributed to unnecessary losses.
If we assume that this year's team will have renewed focus and buy into the team concepts Brad teaches and preaches, I think it's reasonable to think they will meet or exceed what one would expect from a team with this set of pieces. It's not entirely a matter of faith to expect that, because we've seen every Brad Stevens team prior to last season do exactly that. It's hard to remember it now, but the Celts were once known for being a "try hard" team that got greater results than the sum of the pieces on the team should have made possible.
Replacing Horford and Baynes with Kanter and Poirier should make a big (negative) difference on defense. But it may also result in the Celts being a considerably better team on the boards, especially the offensive boards.
That combined with projected growth from Tatum and Brown as well as a return to form by Hayward should make the Celts a more dangerous and balanced offensive team.
The defense should be considerably worse, however. So if you think the drop off on defense will be a bigger deal than the improvement in chemistry and overall offensive potency, it probably makes sense to assume they will have a worse record.
You could also reasonably believe the chemistry problems are not over, that Hayward will never be any better than he was by the end of last season, or that Tatum and Brown will not significantly improve. I think that would be excessively pessimistic, though.
I agree with all those Phosita, and one other factor not really mentioned is that a lot of the teams are worse than last season.
Toronto is the most obvious candidate for a big regression given they lost probably the best player in the game (or at least a top 3). On top of that A lot of their key players are very old (gasol, lowry, ibaka). I would be somewhat surprised if those three all ended the season on the raptors. They probably win at least 10 games less than last year.
Bucks are still good, but still did lose their second or third best player and did not replace him.
Hornets went from mediocre to worst team in the league with the loss of walker and lamb.
Wizards went into last season with Howard, Beal, Wall and Porter and Morris. Seemed like a 45 win team if everything broke right. Now could win 20 games?
Knicks are obviously a complete dumpster fire.
Cleveland probably enters the season with less NBA ready talent than they did last season when they still had Hood, Hill and Korver on their roster. Maybe Love will be healthier this season, but they probably need to move him given he is 10 years older than their building blocks.
Magic are probably about the same. Thought they seem like a young team, something like 5 of their top 6 rotation players were 26 or over last year.
Bulls and Hawks are going to be better from growth of their young players, but are probably still a year away from being legit competitive.
76ers replacing Reddick and Butler with Horford and Richardson is probably a wash or slight downgrade, but simmons and embiid should be a bit better from growth and fitness improvements.
Only teams that really clearly made big upgrades, Pacers and Nets.
Taking all that into account it seemed like we could pick up a win or two from player movement.