"Users don’t know to keep scrolling"—Really?

On multiple occasions, I've spoken about a web design with clients who become concerned that the user will not know that they can scroll beyond a certain part of the website. Invariably, these are full-width sections the client is talking about, but I can't help but think these concerns are invalid because the section in question doesn't even need to look like a footer. This isn't just one client, this is many over various design layouts.

Is this a legitimate concern? Are there actually users out there that don't know they can scroll? If these concerns are in fact invalid, what justification can I give to the client?