I think you underestimate how complex email clients are. Table stakes from Gmail: You need to be able to display arbitrary HTML safely, full text search, remove the embedded history in emails, robust sync with Gmail (or insert your IMAP server), a rich text editor, and there are even small things that end up being complex like parsing an email (people send all sorts of non compliant mail and Gmail is pretty darn good at accepting most of it). That's just table stakes, let alone the features that make superhuman distinct.
You misunderstand me. I am saying that I don't think this user story requires a standalone email client. You could replicate most of the key value propositions (if not the specific feature implementation) as extensions to Chrome and Gmail. That would be both a feasible solo dev idea and a more reasonable $5-10 a month rather than $30.
Sure, throw a couple hundred million at the same problem and you'll overbuild it into a monster. But that doesn't mean it's the right business model.
