So you're thinking the Democrat wouldn't be pulled leftwards by Washington in the way a Republican would? Or wouldn't end up farther left than Thad Cochran? Interesting.
I don't know enough to judge, but 1) the Democrat would help make people who disagree with you committee chairs, and Cochran wouldn't, and 2) while the Democrat might have to take conservative stands to get reelected, if there's a Democrat in the White House, he or she might promise the Democrat a job in the administration if the Democrat senator comes out as a liberal, so you could get four years of votes you'd abhor out of that new Democrat senator.
Here's the dilemma - If you go ahead and vote for Cochran anyway then there wasn't any downside to what they did in the election. That means they'll do it again if they have to when he retires to get their hand-picked replacement installed and you get to look forward to 20-30 years of that.