Rondo being a free agent was not a guarantee to come back, and most likely will be overpaid this summer looking for a max contract.
Thomas OTH is on a great contract for 3 more years, and his contract will even look better in 2016 when the cap skyrockets. We could flip him easily if we wanted to.
Then IMO we got a better package for Rondo than what we gave up for Thomas, not even factoring in yet what Ainge is going to do with 12.9 TE.
That is just off court stuff. On court you can certainly make the case Thomas will be the better player going forward, is a little younger, and has had no major injuries in his past.
Overall IMO the switch to Rondo to Thomas is a slam dunk, and just maybe might be willing to come off the bench in a solid guard rotation.