As usual, you conclude a player is an automation, his talent frozen at the time he's drafted.
Numerous players with great initial talent flatline (see D Brown), others are late bloomers who make adjustments and blossom.
Josh Donaldson spent 2 1/2 years in AAA ball, then hit .241 with a .686 OPS for the As in half a season at age 26. Obviously no future.
Phillies gave up on Brandon Moss, how'd that work out for them?
Now are the odds stacked against Stassi? Sure.
One reason he has to master LF is he nees to show versitility to win a bench spot.
But I have a simple rule, if a player suddenly jumps in performance, it's probably a fluke, if they sustain that jump and there's a good explanation for that improvement (he reworked his swing and his approach to hitting two years ago and has continued to make adjustments), then the odds that it's for real increase over time.
Nor is he a BABIP artifact, his 2014 season he put up a .244 BABIP, after that season his BB% took a significant jump to 13%, his ISO jumped to .170 and stayed there for two seasons, and his BABIP returned to "normal." So his performance the last two years seems to reflect a change in his approach to hitting, he's become much more selective and makes harder contact.