Success is all about network(ing). No matter how good your product is if you don't have network of people to approach and sell, you won't succeed. More people join together - bigger the network, more potential customers. It's harder due internal structure and struggles, sure.
It's ok to be solo founder, it has it's perks. If that is what makes you happy, go for it. But to consider co-founding harmful is simply wrong, it's not what we see in practice for the last couple of decades.
Funny you mention SEO. SEO is good example how network impacts your reach. SEO ranking is all about the network. You can't have good SEO ranking without network (other people and websites referencing to you). So network wins, again. More people you know and share your product in early phase, the better. They will mention it in social, on their blogs etc. If you knew few people, fewer people will knew for you too.
