The Celtics weren't going to have a shot at any of the big names this summer. Just look at how fast guys like Love, Leonard, and Gasol re-upped with their teams old teams. They were never leaving. Players who did move (like Aldridge, Jordan) went to playoff teams ready to compete, Boston wasn't on either of their lists.
We may have had a shot at guys like Harris and Monroe, but Orlando was always going to be able to offer Harris more money, so it just took them deciding they wanted to spend it and we were done. With Monroe, does he really move the needle for us? He would be a legit post scorer but he's not a defensive acnhor and he doesn't change the culture for us, long term.
I'd rather pick up a nice vet on a short term deal, like Amir Johnson, and keep our options open than lock into a second-tier free agent that's going to keep us a solid 5-seed in the East for the next 4 years.