I've been pretty consistent that I don't think Boston pays IT a max contract because it will be a bad contract that will likely cause Boston to lose a young player for a team that isn't a real contender. Now if Boston acquires another player this summer or at the deadline that vaults Boston into true contender status, then signing Thomas makes a lot of sense as it keeps the team at that high level.
What young player are you most concerned about? Jaylen hits restricted free agency during the year that Horford's contract ends. That immediately removes the strain of 1 max contract. Tatum would hit restricted free agency the year that Haywards contract ends. The only dilemma might be regarding the luxury tax and resigning Gordon Hayward. We don't know what the cap will be but I suppose an IT max could cost us Hayward. If Tatum reaches his potential then there is no way Hayward stands in the way of bringing him back. At the end of the day our franchise is fine if Jaylen, Tatum, and future draft picks turn into Stars.
I think you guys are vastly underestimating the cap and more importantly tax implications.
Boston has under contract for the 2018/19 season right now
Hayward - 31 million
Horford - 29 million
Crowder - 7 million
Tatum - 7 million
Morris - 5 million
Brown - 5 million
Rozier - 3 million
Zizic - 2 million
Yabu - 3 million
BKN - 6-7 million
LAL - 5-6 million
BOS - 2 million
So those 12 contracts are 105-7 million, which is already over the cap. That doesn't include Thomas or Smart. Assuming Thomas gets a max at 30 million. That puts the team at 135 million which is well into luxury tax range and doesn't have Smart. So Smart is likely gone next summer because I can't see Boston paying him 15-20 million in salary, when every single penny of that is luxury tax.
So then Boston enters the 19-20 season with 94 million tied up in Thomas, Horford, and Hayward. Brown, Tatum, and Crowder are another 22 million. The 3 18 rookies add another 19 million. So that is 134 million, which will again be over the luxury tax and that is just 9 players and doesn't count Zizic or Yabu (6 million or so) or Boston's 19 pick (3 million). So that puts the roster at 143 million with 12 players. Rozier will be a free agent. No way he is kept. Boston also loses Morris, who presumably is the starting PF. Maybe Tatum or one of the 18 rookies is ready to be a starting NBA PF, but maybe not. So what do you do with that position.
So entering the summer of 2020 when Horford comes off the books, Boston will be a two time luxury tax player on a team that realistically hasn't won a title. Brown will be due for his raise that summer and likely replaces much of Horford's salary. Crowder also comes off the books and Hayward has a player option (which he likely opts out of and then will look for a 10 year vet max contract). Now maybe a team of Thomas, Hayward, Brown, Tatum, Zizic, Yabu, and the 3 18 rookies is ready to compete for a title, but I think the odds of that are low. Boston may be able to avoid the tax that summer, but it means Horford and Crowder are gone and Boston isn't bring any high paid vets and if Boston can't avoid the tax (because of Hayward's raise and Brown's new contract) that puts Boston into repeater tax range and the team still hasn't realistically competed for a title to that point (though might finally have that team as a result of all the young players).
The repeater tax is nasty. It is unlikely that Boston will want to pay the repeater tax, but then that means Hayward is gone and it will be hard to keep all of Brown, Tatum, BKN, LAL, Zizic, Bos 18, Bos 19, etc. going forward.
Maxing Thomas next summer just doesn't make financial sense unless Boston is a realistic contender. I don't see Boston as a realistic contender without acquiring another top level player (which will require some of the prime assets). So if Boston doesn't trade for that player, I don't expect them to keep Thomas if Thomas really wants a max. Even a 3 year max won't fix the luxury tax problems the team will have.