There is one LeBron. There are half a dozen teams who pinned their futures on him choosing them. Most of those teams will get nothing. The Celtics don't need to add themselves to those odds, especially since Boston isn't LeBron's kind of place. He wants to stay home or go to one of the biggest markets. Either way, he wants to be THE MAN, which is hard to do with a historically successful franchise like the Celtics. And since the Cavs have been eliminated from the playoffs by the Celtics twice in the last three years, joining the Celtics would be even more of a "betrayal" than leaving the Cavs for, say, the Knicks or the Bulls. It won't happen.
It's also bad business (long term) to send Pierce on his way to clear cap space after all Pierce has done in a Celtics uniform. The Celtics want a reputation as an organization that takes good care of its players rather than treating them as commodities. That makes the Celtics more attractive to future free agents they actually have a chance of getting.
You know how a teenage Dwyane Wade watched his hometown team separate with Jordan and Pippen on bad terms? And now that he's a megastar, he doesn't know if he wants to go home because he doesn't think the Bulls are loyal?
Penny wise and pound foolish to let Pierce go for economic reasons. He's been the face of the franchise for too long.
I agree that Lebron is an impossibility, so it's not worth cutting ties with Pierce for a .5% chance of that.
However, I don't buy the "loyalty" argument at all. I think it's worse business long term to be signing players to long, over-market contracts based on "loyalty." Players don't really think about the historical loyalty of a team that much, particularly when they are still entering their primes. Yes, palyers are concerned about the skill and committment of ownership, but more in the sense that they want to be sure the owners will pay to surround a star with talent and commit to winning; this is the reason that the big FA's don't trust chicago or the clippers. It's not that those franchises have done wrong in the past, it's that players don't trust them to be competent now.
I think Wyc and Ainge have clearly demonstrated the talent and commitment to build a winning team; if Lebron wanted to come to Boston, the Celts should absolutely cut whatever ties they have with KG, Pierce, Allen, whatever they need to clear to sign him. then, 5-8 years down the road when it was time for the next era, do you really think a free agent would dwell on the fact that some well-past 30 year olds were cast aside to sign the best player of our generation? No. THey'd see a winning franchise committed to putting winning teams out there.