If Irving walks you can't simply "Build around Tatum and Brown".
The Celts will still be over the cap and won't be able to sign anybody of consequence. They'll have to wait until Horford's contract is over and Hayward's is either over or close enough to the end that he'll accept a buyout, before they can even entertain the idea of signing top free agents again. We're talking at least 2 years.
In the meantime they'll have to sign Tatum and Brown to new deals and pay them while the franchise is in a holding pattern.
It's very important that the Celts sign Irving so that he continues to be a contractual asset, if not a basketball one. It's important for Irving too. He'll have a very lucrative contract for 4 or 5 years no matter where he ends up playing.
I'm sure both parties did the math before they agreed to the trade. Remember, he waived a 15% trade clause in order to facilitate being traded to Boston.