I don't have a link for it, but sometime over the past year, the soxprospects podcast reported that there are some in the org that think he could make it to the majors by 21.
I too am skeptical it happens (as were the hosts on the podcast) for the following reasons:
1) While Dalbec hasn't been good, his K rate is actually right where we could realistically want it to be. With his power, if he's whiffing "only" 33% of the time, he's probably worth while starting first base option. The BB rate is a little concerning, but he's gone through similiar (worse, even) stretches in the minors and rebounded back to his normal 10% rate.
2) Cases still has very little experience facing advanced pitching, he's lit up the lower levels and we should all be thrilled (especially given his age) but he's not faced many pitchers who understand sequencing, or have the command to throw league averagish breaking balls on command. The jump from High A to Double A is huge, and it's great that he's thriving in this small sample, but the jump to AAA will probably be an even bigger test for him.
3) The offense has been really good, thus far. If Cordero, Renfroe or Santana can manage to not suck for a prolonged period of time, it has a chance at being elite. If not, they'll likely look to upgrade there.
4) Boston looks like a play off team; they probably will value stability over upside if Dalbec doesn't perform.