There is a tendency, especially in baseball, to believe that once people have shown they can do particular thing, there's no reason to believe they can't continue to do that same thing. At least, until they prove they no longer have that capability.
I don't think the issue was "everyone knew Holland had lost it, except the Cardinals didn't get the word." If you look at the analyses of this past FA market, Holland was rated, by the analysts, rather highly. And a *lot* of players didn't sign until March or April. So, the late sign doesn't support the theory that everyone knew Holland was now horseshit. And certainly the analysts were treating him as if he could come in on any team and take up the closer role.
The reason for the late sign was simply that there were only so many teams who thought they were desperate for a closer, and of the teams who were in big enough need for one that they would blow Wade Davis kind of money on one, well, um... there was only one, and they blew that money on Wade Davis. The only way to get Holland for less money than what Davis got was to make him wait until April to sign. Along those lines, the Cards were likely *ahead* of five or six other teams who were just about ready to make Holland an offer.
Finally -- Holland and Davis were partners, mirror-images, for several years. Davis gets a nice, big contract, so Holland likely figures he deserves similar treatment. When no one bites, and he is forced through the humiliation of waiting until after the season starts before anyone expresses interest in him... what does that do to a guy's confidence?
Yep -- Holland could be underperforming because, head case as most pro athletes are, his self-confidence was shattered when no one wanted to sign him before Opening Day. It could have been a completely different story had a team signed him last December.