If the Celtics trade one of the top players for a pick, not winning next year.
Whereas if we keep 'em we'll win it all next year! Gotcha. 
If the Knicks offered #8 + let's say their 2021 first for Kemba, I would seriously consider that deal. My target would be Haliburton. If he were on the board at #8, I'd probably pull the trigger.
No. Give them Kemba and they have a chance to sneak into the playoffs. The number 8 pick in a weak draft and a non-lottery pick for Kemba? Terrible move.
No one said they would win it if they didn't make a trade, but they are alot closer with those players then a rookie.
No one said don't make a trade. Just that the main return better not be a draft pick for the top 5 players on the Celtics.
Is Halliburton anywhere near as impactful as Kemba as an NBA player? No.
But moving off Kemba can mean it is financially viable to keep Hayward and Smart long term. Smart starts, and Haliburton fills in 20 minutes next year, as a 6'5 clever passer with good length and solid defense, as rotation player.
We still keep other assets we can hopefully turn into one more rotation big at some point.
The team defense improves substantially. The logjam distribution of shots with Tatum, Brown and Hayward goes away. Hayward now feels like he has the right role.
The team gets younger and bigger. The risks of having Kemba lose half a step go away. The playoff defensive hole goes away. We lose an NBA high end scorer, but we going to have shot distribution issues if he stays.
Someone has to go. It should be Kemba.
TP. I fully agree with this. With Walker's knee problems, huge contract, subpar playoff performance and aging concerns I'd much rather keep Hayward. I can see Hayward as a high-end-starter for some more years, but Walker could fall of a cliff or face a lot of injuries. Do we really want to gamble on Walker while he got thoroughly outplayed by Lowry and Dragic?
In a couple of years Brown and Tatum will be even better than they're now, could we not give them then an equally as good as a supporting cast as we had this year? The production of the veterans Walker/Wanamaker/Theis/Kanter together was quite underwhelming.
We could trade picks for some veterans, but it may just be a one year push as we probably can't keep the current core Walker/Smart/Brown/Tatum/Hayward/Theis because of tax implications.
I'm firmly of the belief that Tatum and Brown can become a 1st and 2nd option on a championship team in just a few years. Develop Langford, G.Williams, Timelord and rookies we draft to build a supporting cast from within and create chemistry. Keep the veterans we want to hold on (Smart and Hayward) and trade those who may become too expensive, too old or have not lived up to expectations (Walker, Wanamaker, Ojeleye, Theis?).