Unless someone else can show us otherwise, Yermin Mercedes is controllable for 6 years, according to this link.
As I understand stand it, he would be under pre-arbitration control for 3 years (perhaps 2 years, if he becomes a "super 2").
Thereafter, he would be salary arbitration eligible for the next 3 years.
Then, after 6 years' service on a MLB roster, Yermin Mercedes, at 34 years old, would be eligible to be a free agent.
Therefore, as I understand it, this thread, and the thought of "extending Yermin" is a complete and total non-starter. (Even setting aside that the league has yet to get a good scouting report on him and adjust their approach to him, therefore making it premature to assume that this is what Mercedes will be going forward.)
If anyone can show us differently about Mercedes' path to free agency to a significant degree from what I understand, I'd welcome their input.
Thanks in advance.