He probably is, although he is already showing decline and we cannot know how steep it will become. I cited his decline in oWAR above.
Big point though: I'll readily grant you that he is a good bet for a .360 OBP and .420 SA, which comes out to a .780 OPS, if you'll grant me that a .780 OPS is pretty much crap for a first baseman. His OPS for the last 5 seasons are: .832, .792, .755, .864, .818. Over that period, his oWAR has been 5.3, 3.7, 1.6, 2.9, 2.6.
Of course it's easier to project decline with Santana, because he's been in decline for 4 seasons, while Hosmer hasn't begun to decline yet. I admit that Hosmer's 2017 may prove to be an outlier -- it is by far his best season, but he should be in his prime now, so we just can't know that yet.