I think all this talk about over vs under-ripening is a bit simplistic. Every team wants to promote their young players at just the right time, but multiple other factors get in the way--ELCs, competition at the position in question, unwillingness to put a veteran on waivers, the need to 'win now', physical immaturity, etc.
Looking at all those factors, the Jets tend to err on the side of caution and long-term development, which I support in general.
I'd give Cole another year to physically develop and really separate himself from the AHL pack. If he is 'tearing it up' by the end of season, then call him up as needed.
Heinola is ready now, but suddenly there is no room, which is a shame. I am, however, comforted by the fact that he will be an injury call-up and none of our other D was playing regularly in the NHL at the age of 20. Looking back >10 years, Trouba and Bogosian are the only exceptions I can recall, and they were both physically much more mature than Heinola. (I'll bet, in retrospect, Bogo wished he had been more mature in other ways.)