Like @TheDoc, I’m very much a front end guy. But I often find myself wanting to build simplistic code for my needs. GitHub doesn’t always (often, not always) offer a great solution. Usually I can use logic to figure out what’s going on and make some modifications, but they’re not nearly as extensive as I wish they were.
WordPress seems like an overkill with most websites I come across that use it.
I’d agree but you are also negating the fact that WordPress from consumer perspective offers a lot of options. From theme selections, to plugins, to availability of developers to hire, just overall resources make it the “go to” platform. At the end of the day it solves problems for many people.
Is there any real downside to not having a database for a smaller site?
As @Joe_Temp said, you either need one or you don’t. There’s a small overlap (as you mentioned, very simple content management) where you might make either choice.
The real advantage for Kirby isn’t that it uses flat files instead of a database, it’s that it is largely self-configuring and doesn’t require a web host that supports MySQL. It’s convenience. (Not a bad thing.)