As a developer who have used both and now uses primarily Kirby I think I can contribute to this.
The points made by @lukasbestle are indeed valid. Kirby doesn’t need lots of updates and maintenance.
You can simply keep an eye on the releases and update it every now and then. Also, since you don’t need to deal with the craziness of the WP ecosystem you can build much more stable websites. Plus, Kirby is not an open source project, the direction they’re taking is clear and the communication here is great so you know what to expect moving forward.
With WP, that’s not always the case. Plus legacy code. WP has been around for a looooooooooong time and as much as I appreciate this philosophy of not wanting to break anything and always have backward compatibility is starting to hurt WP itself at this point.
If you’re dealing with a client who says they want WP because someone knows how to use it then you can show them that Kirby is as easy if not easier to use. You can really customize the heck out of it and make the admin with only what they need (particularly exited for what’s coming in V3 btw).
I must say that WP generally gets bad rep because of how most “developers” use it: they install a theme, a bunch of plugins and call it a day. That’s certainly one way to use it. If instead you know your way around it you can use it as a normal CMS, code a theme from scratch and use almost 0 plugins.
I coded many WP sites using essentially only 1 plugin, ACF, and that’s only to make WP do what Kirby does out of the box (almost).
One HUGE plus for Kirby is the multi language support.
Also, depending on the size of the project, WP can be a total overkill and be quite slow.
So yeah, I think the best way to convince someone is to show them what Kirby can do and show them the admin.