I'd actually be in favor of keeping the numbers in the rafters as they are, but letting current Celtics use the numbers in an honorary system.
33 is the Larry Bird
5 is the Kevin Garnett
34 is the Pierce
0 is the Bill Russell.
To wear that number the Celtics brass have to vote on it. Another option is that the owner of the retired number can delegate that number to whoever he thinks deserves it. Make a big ceremony at the start of the season...use it to sweet talk free agents etc..
if Bill Russell came and said to Anthony Davis, we want you to wear my number, youre the only one good enough etc...that would be quite an honor.
Those names will forever own those numbers, but they may also have co-owners if someone's good enough.
If Kevin Durant wanted number 34, and Pierce is cool with it, then boom.
I do see the other side of the coin though.