Pence, Lance, Lee are locks and not needing upgrade. There is nothing in the SS market better than everett that they can afford, and they have decided their catcher plan.
That means you can upgrade at SP(nothing good in this market), RP(linebrink would be great) 2B(as I said one of iguchi, loretta or castillo will be the OD starter), 3B(A-Rod ain't happening, and lowell is going to get huge money, plus wigginton was traded for to start at 3b) and one OF spot(tons of good CF options available in rowand, hunter, jones, crisp, cameron)
Also consider this is your rotation:
Roy and a combo of sampson, backe, wandy, gutierrez albers, patton, paulino, woody. After roy it's all about average at best. Also consider you have a thin farm system and no assets you can spare to upgrade the rotation via trade, plus how many stud SP's will ever see FA, not many. Santana won't see it next year, just like zambrano, etc.
If you want to win, and the astros do, you can't go into 08 with this pitching and that defensive outfield. You will see mediocre pitching results and the offense isn't good enough to overcome it. Now replace scott with hunter, rowand, cameron, jones, and perhaps crisp(he seemed to improve defensively in boston) and you lose nothing offensively if not potentially gaining a more consistent middle of the order guy(other than crisp who would fill the leadoff/number 2 hitter moving pence to 5th) but your outfield defense goes from below average overall to very good in one move.
Add a good defensive 2b like castillo or iguchi and your defense suddenly is in the elite, and can mask your average pitching, turning a blah team into a good one in one fell swoop. Not to mention it frees up scott as an attractive trade chip for the pitching help you need.
You can say it's not worth 10 mil to go from scott to one of the good CF's on the market because scott's OPS and theirs are similar, but that's foolish and not looking at the big picture. Your pitching isn't getting better anytime soon as the astros don't have the next Roy in their system right now, so eventually you will have to fix that OF alignment if you want to consistently win. And scott will go at that time as pence and lee are cornerstones of the franchise.
To get the complete OF they desperately need will cost 10 mil in any market, you might as well spend it now and make scott tradeable when he has good value, instead of waiting and potentially watching his value decline. Plus, you better your odds of winning and making your franchise an attractive free agent destination in the future. If you leave that outfield as is right now, with this staff, another losing season occurs, and free agents don't like playing for bottom dwellers.
I understand what you're saying.
I agree that the defense would be improved, that's pretty obvious. I just dispute that the specific improvement that you're advocating is a necessary condition for the Astros having a winning season. I know I'm alone on this, but I think Burke has a better than average shot at being a huge upgrade defensively and offensively at second over 2007 production. Other things that have above average to above average chance of happening that would improve production over 2007:
1) Berkman rebounding from his worst season since his rookie year
2) Towles posting a better OPS than mid .600
3) Pence playing a full season
4) Scott playing a full season
5) Wandy improving consistency on the road
6) Backe for a full season
7) Significant progression of any of these: Patton, Albers, Gutierrez, Nieve, Sampson, Estrada, Paulino, Barthmaier
Basically I think the Astros have enough to be competitive in 08, enough to play with what they already have. Better to save the money, especially considering the potential free agent class next season.