A pick like the suggested Lakers trade netted us an All-Star (hayward) it also netted Ainge the two all-stars he built his last team around... I'm guessing he very much wants those picks.
There is no point to retaining talent that will hurt the tank and won't be under contract in a year or two.
We are seeing the difference in trade value for unprotected picks and protected picks right before our eyes in the Knicks trade. We'd rather have the far out unprotected pick than the protected picks (they'd willing part with).
Even if you don't wait the full 5-7 years... those picks have a ton more trade value. So if the core of this team is ready to go in 3 years you can use those picks to add something important.
Why would we gain an extremely valuable asset and help our tank when we can retain talent that hurts our own pick?