The most ignorant client is the one requiring you to build his crapsite on WordMess despite of you showing him the glory and beauty of a proper CMS which has a brain-amputee-friendly back-end.
Some people just want and need constant pain on their servers.
His reason to prefer WP:
"I know how to use it."

War, war never changes...

He has a valid reason. Learning another system would cost him time, which equates money, and he probably has other activities with more ROI on his list.

@Fast-Nop I understand your point and it would make sense in a lot of scenarios, but...
in my experience with customers using both CMS, they have only needed an average of 30 minutes from seeing the Craft back-end for the first time to being able to independently create and edit content.
With WP it took me mostly more than 1 hour to school clients in using the WP back-end but they keep getting back with questions and problems.
WP is a big UX mess.
After all, I rarely get any emails from customers with problems when they have Craft, which in my view shows the huge ROI by changing to Craft.

@PonySlaystation That may be right, but misleading. If the customer already knows WP, he won't need additional time to cope with it. Changing the system would require additional time.

If you want to be convincing, you need to point out tangible benefits of the change, not just dev techno babble that doesn't mean shit to users. Which is also why more mature companies don't let devs talk to customers.

Also, it's no wonder that the customer doesn't believe the "but it's easy" because most of the time, it's just not true, especially when it comes from a dev.

@Fast-Nop Let's say it was more of a decision of my boss to give them what they wanted instead of showing them the proven benefits of switching.
Even as a dev, I could have shown them the benefits and time savings they would have had, without mentioning any tech. advantages.
I have yet to see a CMS that has better back-end UX than Craft...