While the first Mega Man game featured an infamously terrible cover art illustration, that’s not what almost killed the series.
According to EventHubs, the higher ups in Capcom at the time were none too keen on the underperforming Mega Man getting a sequel. However, the game’s director, Akira Kitamura, remained vigilant and eventually convinced his bosses to begin development on the sequel. But there was a catch: Kitamura and his team of developers (which included Mega Man designer Keiji Inafune) could only work on the follow-up as long as they put their efforts towards other projects at the same time. In other words, Mega Man 2 was made by a team that seriously had its hands full.
It turns out that decision paid off handsomely as Mega Man 2 released to rave reviews and high sales (1.51+ million copies sold). And because of that, Mega Man got to fight on and bring in the big bucks for Capcom ever since.