Well, neither of them were meant to take over in LF. Marsh, as a LHB platoon player, was always going to get as much as 75% of the playing time in LF, and Merrifield actually made more appearances as an IF (27 between 2B and 3B) than as a LF (25).
And while they did decide to shoehorn Hays into additional playing time instead of merely letting him be the RHB platoon guy that they needed, that came at the expense of Rojas. And Wilson outperformed Hays in the role anyway.
Because Kepler is a LHB and not a bad fielder, he's a good pick-up, but he's not an upgrade over Marsh in LF per se. It's just that Marsh is an offensive upgrade over Rojas in CF, and they are willing to have a worse defense.
Stubbs is only gone because Marchan didn't have any options left. Would have been bad business to trade the much younger player or lose him for nothing. But he didn't beat out Stubbs in spring training.