I still like MAT. Hits better and plays better defense. Fast...might be a smarter base-runner. Yes, Taylor's hitting has fallen back to about .275 and .800 OPS, roughly his good 2017 season. For now, Stevenson should start.
Thinking back, I don't remember that Robles fought for and won the starting CF job. My memory -- not entirely trustworthy -- is that Robles was given the starting job.
I think what cost MAT his starting position was his 2018 after his 2017. 2017 looked like he had clicked with Dusty, but he reverted to 2015/2016 levels (.230 AVG with mediocre power). I think in 2018, Robles was ticketed to be the first OF called up, but was hurt when they needed an OF early in the year, and Soto was called up. Robles got to play late in the year. More or less, Taylor wasn't playing well enough to keep them from trying the hot prospect when he was healthy. I don't recall if they split time or it was all Robles at that point.
Robles's offensive numbers were actually very good in 2018 over a small sample - .288 / .348 / .525, with good contact (18% Ks, 35% hard contact) in 66 PAs, so that was good enough to make him the presumptive CF going into 2019. He showed adequate offense and great defense that year, which more or less doomed Taylor here. Recall Taylor was sent down in 2019 and kept down once Parra was acquired?
Offensively, there were big warning flags for Robles in 2019 in terms of strikeouts and drops in hard contact that I think were ignored given facially acceptable good average (.255), OPS (.745), steals (28), and exceptional defense. In 2020 and 2021 so far, many of those warning flags turned into a reality that arguably is worse than the contact rates project to. The contact is ridiculously soft, but he's had a huge spike in BBs and a drop in Ks so far. Meanwhile, Taylor is back to 2017 levels.