Other than a top ten pick, It typically takes years to develop a good DT in the NFL. Any replacement at that position would be a downgrade.
One of the greatest issues the Cowboys have faced the past decade has been their inability to keep their lines deep. Their frontline guys have been pretty good but when they go down, the decrease in talent is significant. It would be great to back up the DT starters with a high draft pick like we see at DE
You can never have too many good DT's. However, their average career length tends to be shorter than other positions. This is the reason you see the hefty price tag.