Brought this up a few weeks ago:
Just took a look at what Pierce did in the 8 game stretch with Rondo out and it seems to back that up:
1/20 (LOSS) 12 points, 6 assists, 3 rebounds, 2 steals, 2 blocks
1/22 (Win) 34 points, 10 assists, 8 rebounds, 3 steals
1/23 (win) 19 points, 7 assists, 5 rebounds
1/26 (win) 24 points, 10 assists, 6 rebounds, 1 block
1/27 (win) 28 points, 8 assists, 10 rebounds, 3 steals,2 blocks
1/29 (loss) 18 points, 5 assists, 6 rebounds
1/31 (win) 20 points, 6 assists, 4 rebounds, 3 steals
2/1 (win) 17 points, 8 assists, 6 rebounds, 1 steal, 1 block
4/18 (win) 29 points, 14 assists, 5 rebounds, 2 steals
Pierce usually steps up his game with Rondo out... which makes sense since the ball pretty much flowed through him his entire career up until late 2009.
Our offense was *worse* when Rondo was out for those games, not better.
I should hope so... otherwise Rondo is the most overrated player in the league. Still... we usually manage to cope without him mainly because Pierce is a better player and he steps it up.
Ah hem... I rest my case.
5/1 (win) 36 points, 14 rebounds, 4 assists.
I'm not sure why it should be surprising that PP has big games when Rondo's out. He's the only other player on the team that can really handle the ball or create anything on offense. What do people expect to happen to Paul's numbers when he has the ball in his hands every trip on offense? This isnt the unlikely occurrence that you think it is.
Point is... it makes you wonder if our offense would actually be "better" by replacing Rondo with a shooter like Steph Curry or Jose Calderon. As long as Paul Pierce can carry the load on offense and is comfortable letting the ball flow through him... you can easily make the argument that having a shooter (curry) as opposed to a setup guy (rondo) would be more beneficial to our offense.
Naturally you remove Rondo (our 3rd best player) and we're going to take a step back... but funny we always tend to get the job done without him. Alternatively... remove KG or Pierce from the lineup and the team is pretty much always toast.
If the argument can be made that our offense would be "better" with a shooter like Calderon... surely the argument can easily be made that our offense would be better with a complete offensive star + distribution star like Deron Williams. 
Your argument is very peculiar. I thought part of the point was that before long we'll have to replace Paul Pierce and Kevin Garnett, regardless of who our point guard is. It seems that in your fantasy world, we replace Rondo with Jose Calderon or Steph Curry, or even Deron Williams, and, then, Paul Pierce and Kevin Garnett play at an all star level for another 7 to 10 years.
Or, are you still talking about upgrading for this year? This is just a reminder; the trade deadline has passed.