Rosberg was more than capable of mounting a title challenge where the only other competitor was his teammate, the worst result he was going to get was 2nd. Hamilton was in a two person fight where he had to finish first or lose the title. Pressure, add to that Rosberg was more than handy.The only opposition in '14 was Rosberg, who is a great driver, but it's a step below Hamilton. It would be the same if MV would win a championship vs DR.
'17 Vettel completely threw it away with stuff like Baku and Singapore. The Ferrari may have been the faster car at quite a few points, but the whole operation (team + driver) just self imploded. In 18 Vettel continued with making mistakes (Azerbaijan, France, Monza, Hockenheim...).
So Lewis was facing a tough and fast car, but he wasn't really challenged consistently because the other guys dropped the ball massively. Not Lewis' mistake, but let him please finally have a fight until the last few races against a driver who can match him in speed and racecraft.
As for the fact Vettel blew it, it proves my point. A more than capable (in fact at the time considered one of the best in the business) blew it when he had two shots at the title against Lewis while Lewis was near perfect. That shows his talent for me, not the opposite.