The evidence in clinical trial evidence is that nicotine is not particularly hazardous. It's not associated with an increased risk of cardiovascular disease. It does things to the human body that are probably on a par with caffeine.
The best long-term evidence on the hazards of long-term nicotine use without combustion is the evidence from exclusive users of smokeless tobacco in Sweden, who do have a possible increased risk of esophageal cancer and cancer of the pancreas, though those things may well be due to nitrosamines in tobacco that they're swallowing.
But the other disease risks—lung cancer, COPD, and cardiovascular disease—are just not substantially increased. For cardiovascular disease, there's a slight signal. For COPD and lung cancer, there is nothing.
I think we can conclude from that experience that the hazards of regular nicotine use to a healthy person for a lifetime are, if not trivial, close to it.