Doesn't add up.
If I recall, the Celts needed Horford's cap hold freed up to give Kemba the max.
Boston could have theoretically kept Horford and then sign and traded for Walker. Boston just had to remain below the tax line to do it, so something like Rozier (sign and traded) plus Smart, probably would have worked financially. Still might have even been able to add Kanter depending on what Horford's contract actually looked like.
So is a Walker, Brown, Hayward, Tatum, Horford with Kanter, Theis, Langford, etc. a better team. Probably, but it still isn't a contender, so I again am not sure what the point of all those moves would be. Of course I have no idea why Boston signed Walker and kept Hayward, if the goal is to build a contender around Tatum.
That team doesn't look like a contender when compared to the NBA since 2008 with loads of superteams with 3-5 stars per team.
But this year's NBA isn't made of superteams. At best you have teams with 2-3 stars on them and some of those teams don't have great depth or have injury problems.
Then add in that there could be even more injury issues and, as far as I am concerned, you could argue any one of 6-9 teams could be the champion if they remain healthy.
A team of
Kemba
Brown
Tatum
Horford
Kanter
Edwards
Smart
Utah Hayward
Theis
Williams
Williams
Semi
That gelled and stayed healthy could have been a contender in today's game with the talent more evenly distributed.
I'm confused, who are these teams that had 3-5 stars on them? Sure the Warriors, but after that, what are we talking about recently (I'll give you the Heat and C's before that, but the heat were 5 years ago - perhaps the Cavs when they had Irving). The Clippers and Lakers this year project to be better than any team not named the Warriors (when healthy) the last 5 years. The Rockets are probably better with Westbrook than Paul (at least in theory). The Nuggets, Jazz, and Blazers are all better than that Celtics team. Heck if Klay comes back, I'd take the Warriors over them in a playoff series. The East is weaker, as it has been for years, but that team is pretty clearly behind the Bucks. The Sixers are harder to gauge because in that scenario they don't have Horford, but they would have used that money on someone and they still have Embiid and Simmons so I'd rate them higher. Obviously that team is better than the Nets on paper this year, but not going forward. That hypothetical Celtics team is at absolute best, the 7th best team in the league this year and they are arguably more like 9th or 10th. That isn't a contender.
I love the enthusiasm and the positivity so many have, but the C's aren't close to contending. The team just doesn't have enough high end talent and the roster construction is terrible. Even if your hypothetical team was somehow possible (I think it is impossible as Smart couldn't really be on the team unless Horford took a very small contract), it isn't a contender either. Better than the current group, absolutely, but just not in the same class as the elite teams, because quite simply the elite teams have elite top end talent, something Boston does not (Walker has 1 3rd Team All NBA, the only such award from anyone on that team - and I don't see Walker as a top 15 player despite him having a top 15 season last year). Top end talent wins. This has been shown time and time again that the team with the best player in a playoff series wins that series at a significantly higher rate than the team without the best player (there are exceptions obviously).