I'm no "roster guru". I just track what's posted.
Thuney played significantly better at Center the one game than Andrews ever has. I disagree with your downplaying of how important a center is. The Center is the GLUE of the O-line. And Thuney can handle the Big NTs that Andrews can't. Also, Thuney has shown the ability to stop the stunts/late blitz up the center that Andrews fails to catch on a regular basis.
I contend that Center is the 2nd most important position on the O-line behind the Blind-side Tackle. I also contend that with the Cap space they have, signing Thuney to a 12M deal is just the thing they need to do to ensure continuity on the O-line. In fact, Thuney would be an improvement at center. While Onwenu is a slight drop at LG over Thuney now, in the long run, he should be just as good.
The days of paying O-linemen the least amount on the team are gone. And teams succeed based on how lines are built. If they aren't willing to keep an All-Pro Caliber lineman, then they are taking a step backwards on the whole.