Their 2017 statistics were very similar, almost exactly the same yards per attempt. Keenum had a better supporting cast too. Cousins had a higher touchdown rate but also a higher interception rate. But up until last year, Keenum hadn't done a damn thing in the league while Cousins has had three straight seasons with 4,000+ yards, 25 or more TDs, and 13 or fewer INTs. Cousins also had a terrible offensive line last year and took over 40 sacks after taking around 20 the two previous years.
I don't think Kirk Cousins is a worldbeater by any standard but I do think he's likely to be better than Keenum in the long run. Frankly, I think the Vikings would have been better off keeping Bridgewater on the cheap than going after any of the expensive players on the carousel.