The most important stat for a hitter (at least basic stat) is not average, and it sure as hell is not RBI. It is OBP. Desmond's OBP is .295, meaning he makes an out more than 70% of the time he comes to the plate. That number is ATROCIOUS. There is no way to say it, it is probably the worst number in the majors for an everyday player. If not the worst, one of the five worst.
He has certainly provided pop this year (which for the umpteenth time, I have no reason think will continue), and because of that he has made himself more valuable to date. For example, at the link above look at the stat wRC+. This number is "weighted runs created" adjusted for ball park factors. This takes into account everything a batter can do with the bat and normalizes it to a score. The score of 100 means he is an average major league with the bat.
Desmond's defense has been good. He does not have "ridiculous range" and a "cannon arm." He has good range and a good arm. He is certainly a capable defender at SS.
RBI is essentially a worthless stat. Average is not worthless, but it is not nearly the best thing we have. Slugging is much more important compared to other "basic stats," and he is excelling there now. But its not going to continue. I am certainly not going to cry if in September someone moves Espinosa to SS and Lombo to 2B, or even the reverse. I am definitely not going to cry if one day we have Lombo at SS and Rendon at 2B.
A SS who plays good defense and is an average hitter is definitely valuable. However, he is not a super hero. And again, unless he keeps slugging like he never has before in his life, that number is going to go down. At which point he'll become an average SS.