Hindsight is always 20/20.
That said, it was the right decision. At the time of his signing, if I recall, he was ranked #20 in the NBA by ESPN (even higher than Kyrie at #25.)
Like someone else mentioned, Ainge didn't foresee Tatum making such a quick impact and Hayward getting seriously injured on his first night.
Believe me, I've been very frustrated with Hayward this year and feel he got a pass while many were so quick to jump on Kyrie.
That said, with another summer of training and working more on his game and less on rehab and hopefully with no logjam of small forwards, I expect and will be rooting for a much more improved Hayward next season.