All your plan does is sign us up to bottom-tier playoff mediocrity (6-8th seed) for the next 2-3 seasons.
In other words, it just wastes years of Rondo's prime while putting off the necessary rebuilding effort. If we're going to keep Rondo and we want to turn this team into a contender again before Rondo is in his mid-30's, the rebuilding process has to start as soon as this season is over, if not sooner.
I think this team is at a crucial cross-roads that turns on one key question: Keep Rondo, or don't keep Rondo?
If the answer is no, then you either completely rebuild while trading him for assets that help you do that, or you trade him for pieces that will better fit with the Big 3 and help you win now.
If the answer is yes -- and all things considered I think it is -- then you look to start rebuilding with an eye to acquiring players that are young, athletic, and good at getting out on the break. In other words, players that will fit Rondo's style. Rebuilding with Rondo will still require that the team gets a true #1 guy who can carry the team offensively -- in essence, the player Pierce once was (and still is at times).
Any way you cut it, there's no sense in holding onto the same core group and surrounding them with complementary pieces, because at this point they can't compete talent-wise with the best teams in the conference -- and they're declining more every day. Furthermore, as the Big 3 get older, they get slower and less athletic, which means more and more the style of play that suits them best is opposite to the style of play that fits the team's best player -- Rondo.
The only reason that any team ever has to keep the core group of their team the same from season to season is either that they believe the core group is going to improve over time, or that they truly believe they have a chance to win a championship as long as they can surround that core group with the right pieces. At this point, the Celtics have a core group that is only getting worse over time, and that I think most would agree is no longer capable of true championship contention, particularly now that the East is so much stronger at the top than it used to be. Hence, it makes no sense to hold onto this group after this season.