I have wondered about the year of death and posted about it before. The time of the killings was estimated to be around 1977-1985, but with the discovery of Evans it narrowed to the latest being 1981. But did it narrow more? Is the late 70's not as probable anymore? IMO I think it's important to not discredit it until we have solid proof they weren't killed earlier, because, like you said, it would put their DOB's at an earlier time and someone may have a lead and it could get overlooked because of the differences in time-frames.
I feel like people are looking more toward 1980-1981 now because of Evan's timeline (his work in Bear Brook State Park, his sudden disappearance, etc.), but I'm not opposed to their deaths occurring as early as 1977. That is the earliest the whereabouts of Evans was known, and he was in the area. He could have moved to Manchester with the victims, or they joined him shortly after. No one remembers them because Evans is new to the area. I think another reason for the assumption that they were killed later was the Carol Cable wires found with the bodies, and Denise being employed there. I see that being circumstantial evidence, but at the same time, with Evans being an electrician since at least 1977, he possibly was shopping at Carol Cable long before he even met Denise.
Also another thing I've thought about. Eunsoon's body was found hidden inside their residence. What if this is what Evans did before? What if he hid the body/bodies where he knew they wouldn't be found, and then when he was ready to skip town to another alias he dumps the body/bodies? Could Evans have concealed the bodies and dumped them shortly before leaving in 1981?