I don't really know what to make of the apparently widespread and statistically confirmed notion that Contreras doesn't work well with pitchers, and then him getting a big contract with the Holy Citadel of Catcher Defense.
Contreras' value isn't really all that tied up in defense. He's not a great pitch framer, and not known as a great game caller. That's all part of the reason his trade value cratered. Having the off-season to work with a pitching staff will help, rather than having him figure out pitchers' strengths and tendencies in the midst of a pennant race. Like he did as a Cub last year, I assume he will DH as often as he catches.
When a team has an all-world defensive talent behind the dish for 18 years, it's hard to imagine them not having that continue. Molina's tenure had 3 different GMs... Mozeliak never had to even think about catchers...
I don't know if the Cardinals value defense over offense at that position, or they just so happened to luck into Mike Matheny in 2000 (Surprisingly made the team out of training camp), and subsequently had the best defensive catcher of a generation take it over for the next 18 years.
By luck or design, it's going to be a different experience for Cardinal pitchers the next few years.
I tend to think Molina was the secret sauce that has kept the floor so high for them over the last 20 years. Their pitching staff is not as talented or deep as what they typically enjoy. Losing Molina's guidance, experience, and defense won't help.