Only if you believe his junior year was more indicative of his potential than his sophomore year. Lot of people do not believe that to be the case, hence why people wanted him to come back so he could return to form and improve. He would have another year of competition, weight training, practice & game film to review mistakes, and coaches who are invested in him.
I think he was much better as a Sophomore, when he had 5.5 sacks, than his Junior Year, when he had 5.0 sacks. That's why I don't think a huge leap is a given.
Competition can sometimes work against you.
Trevon Hill had 3.5 sacks in 3 games his Junior Year. I'm sure he didn't see making 1 start and getting 4.5 sacks in 12 games this year as how it would play out when he transferred here. But Rousseau blew up and took that starting spot away from Hill. Hill didn't improve his stock at all.
Same thing could happen to Garvin next year. Rousseau has 1 DE spot on lock. What if Phillips is the real deal and takes the other DE spot? Garvin could be better next year, but still lose his starting spot, which certainly wouldn't help his stock at all.
I think the pros of Garvin returning are better than him going pro this year. But I think injuries, loss of playing time, leveling off or regressing are possible scenarios that need to be weighed in when making a decision.