There are a few strategies that you can take at the trade deadline.
First, if you are a team that feels you have a shot at a title or a deep playoff run, you might go all in on a veteran that you feel is the one piece needed to get you over the top. Maybe you give up a good young player or a valuable pick but you get back a solid vet for a short term fix (rental).
Second, you can do what Portland is doing, full reset. Everything goes but one or two key pieces (if that).
And finally, what the vast majority of the teams do, you tinker with the team to fill a need or to manage cap but don't do anything that is a long term negative for the team.
Some bigger deals that are not "all in" type or "full rebuild" type do get done but more often those intermediate deals get done in the off season.
Do people really want to the Celtics to go "all in" on some vet rental that may or may not help them win even one play off series? Do you really want to Celtics to go nuclear and trade everyone not named Tatum or Brown? I think they are doing exactly the right thing. Stay the course. This team has a good solid foundation do no need to blow it up and we also aren't quite a the point where a key vet rental is going to make a difference.