You employ a different form of logic than I do or any MLB team for that matter. No team is going to make a retention decision based on a half-season performance measured by a single statistic for a player with over 2000 PA over 4 seasons. I simply pointed out that his 2nd half was a bad as his first half was good.
I would make personnel decisions based on a larger sample size but I would give some weight to the most recent data. Rosario’s career OPs is 35 point below the league average. His OPS this year was still below league average. Apparently your logic dictates that focusing on the bad outlier is cherry picking but focusing on the good outlier (1st) half should be the basis for personnel decisions.
My entire point is to quit focusing on the outliers. Don't cherry pick samples either way. Don't pretend like baseball teams' front offices do, either.
Also, I'm not sure where you're getting your stats, but Rosario's career OPS is .784. League average for the last 80 years has never been that high. Similarly, Rosario's 2018 OPS is .803. League average for the last 10 years has been around .720.
Also, it so happens that Rosario has, beginning in 2017, dramatically improved his discipline. He went from a strikeout to walk ratio in the 7 range in 2016 down to 3 in 2017. He regressed a bit this year, to 3.5, and I don't personally see that as any reason to panic. His walk rate this year is above his career average, and at 26 years old, I don't know why anyone would assume it has peaked.
Finally, you think you'll find a team that will trade for him on the basis of his .950 first-half production, and not on his career numbers and trends? That's ridiculous, and I don't think it lines up with your claim to know how baseball front offices make their decisions.