I don't think he's in a slump, nor do I think he is declining as the season goes on. It's clear he is physically not the player he once was, but his decline in stats this year are directly attributed to a new offense that no longer goes through him and a calculated preservation of his energy and minutes. Duncan is in Horry mode and the Spurs are good enough to allow him to coast.
Every few games he reminds you that he still has it. You get used to him never leaving his feet, mistiming blocks and rebounds and being step slow on rotations, but all of a sudden, he will look like the old Timmy. Spry and bouncy, aggressively pulling down boards and blocking shots, hustling down the floor, shaking off defenders in the post . . .
We saw him do this last year, albeit with a much less talented team that required more production from him, but as Benefactor illustrated, you can't count out Tim Duncan. It isn't wise.
I am, hopefully, voting for the third option.