People are talking about his swinging strike rate like it will never change. Firstly, his swinging strike rate is based off 125 innings. His velocity as a starter was down last year. His command certainly needs to improve. The addition and improvement of other pitches will help make his fastball better. He's primarily thrown his fastball so hitters thus far in his career. Hitters have thus far just sat fastball and with the command not being very good in those 125 innings you are basing assumptions regarding contact rates, which are higher than you would like, on a limited sample size. The benefit of the movement can not be denied. He has thus far had an elite level ground ball rate.
Determining whether he can improve his fastball command and develop his cutter, change, curveball isn't something that you can do simply looking at statistics. Is his delivery showing improvement this year? Can't use statistics to make that analysis. You only have 9 innings to go off of.
His curveball seemingly looks better, but he hasn't thrown enough of them to form a conclusion. Sanchez has the foundation to become a great pitcher. There just aren't a lot of starters who can throw 98 with movement. He may or may not develop the secondary pitches and command with is fastball to achieve long term success, but putting percentages on him becoming a number 1 or 2, or a 5th starter, or reliever is just wild ass guesswork as well. Historically power arms often take longer to develop and some develop and some don't.