I guess it's better to go with the sure thing than to hope you can sign Hayward or Griffin. But it's going to cost them the #3 and then some, plus take up that max salary slot.
Butler only hits them by $18.7mm. After waiving Zeller for nothing they'll still have just under $20 million of cap space. They can also free up about $3mm more with unguaranteed deals. There will be suitors for Bradley and Crowder too that can free up to the $30ish million max.
So could be looking at Thomas, Butler, Hayward/Griffin, Jaylen Brown and Horford with Bradley, Smart and Rozier off the bench (will need more size)