One possible good sign was his significant increase of power last season, doubling his HR and ISO. His FB% increased 5%, which doubles as a power increase and less outs on groundballs with his lack of speed. I project him with a .285/.340/.412 slashline, about 5 wRAA. This leads to a 4 WAR season, but the extension doesn't start until after this season. Here's my timeline of production under the extension:
As you see, with 3% inflation and the standard .5 WAR a year decrease in value, Molina falls short of the $75M value. This does not include the $6M buyout on the 2018 mutual option, further increasing the overpay. The lack of production should not be surprising, since only eight catchers have ever accumulated 15 WAR in their age 31-35 seasons.
Not only did St. Louis drive up the price of their own catcher, but they helped Brian McCann, Mike Napoli, and other catchers nearing free agency. The Cardinals do not have a major catching prospect in their system, so they were willing to spend a bit more to keep Molina. There have been much worse overpays, but any overpay is a bad one.