You mean like Charizard, Mewtwo and Lucario? Perhaps you could count all version mascots (besides Yellow) as well...if people are turned off simply by the game's mascot that's their own close-mindedness keeping them away not the games themselves...its like saying 'I don't want to play Zelda because I don't like Link's hat!' Although they'd probably call him Zelda because they are ignorant...
However, as stated with the whole reboot theory making the games appeal to an older crowd in addition to children is something that possibly can entice both new and former fans to come into the series...
TBH with a reaction like that I'm not sure if they even knew what other than Pikachu even existed. And considering Pikachu is disproportionately on every piece of marketing compared to those three and the mascots, on the vast majority of merchandise. . .it's not a "Zelda's hat" situation because Link is the only playable Zelda character and there are 700+ other Pokemon.
And what reboot is exactly going to catch people's attention? What "dark storyline" is going to get people not in the fandom interested? Turning and leaving because they see Pikachu as the face of the series may be shallow, but humans are pretty shallow in general. Once they saw that face, if they deemed his design "kiddy" they wouldn't even stay long enough for the plot.
Heck, I'm fine with Pikachu. I just know a personal example of someone hating them to the point of wanting to punch them in the cheeks (at which point I pointed out Pikachu's cheeks are where it's electricity comes from and they said Pikachu wanted to be punched then), and I get the feeling like that might extend to a lot of other people. A co-mascot would do more for their image than any grimdark reboot, because at the end of the day it's still the series with the weird yellow mouse to anyone who isn't specifically paying attention.