How lucrative is the email client market? Could the makers of Sparrow have survived for a while? I would think that a lot of people are already satisfied with M

How lucrative is the email client market? Could the makers of Sparrow have survived for a while? I would think that a lot of people are already satisfied with Mail or a web client, and if someone bought a copy of Sparrow they wouldn't be buying another copy for a while. I'm surprised that there are multiple companies out there selling Mac email clients.