Your forgetting that this favors your more than them.
People dont want to learn a new system, and they are confortable with wordpress.
There are millions of devs and designers also familiar with wordpress - which can mean they can leave you in an instant and get anyone else to make changes or add feature to the site easily.
That's assuming they are already familiar with wordpress, most people have never used a CMS in their life.
If requirements are simple then all you generally need is an admin area with about 3-4 links.
Something like:
Change Homepage Text
Show Customer Inquiries
Add Item to catalog
This is much easier to understand to a newbie than "Add Page" , "Add menu item" etc.
If they want complicated changes they will usually end up calling me anyway.
I always keep the coding for my simple sites simple enough that any competent developer should be able to figure it all out in an hour or so anyway.
Many people who are not technically inclined do not have time to do much modification to their website themselves, so will generally not bother if you give them something with a lot of power like a full CMS.
Most small business people aren't going to go around installing new designs on their site on a whim (isn't that what they paid the designer to design?).
They want something they can log into once every couple of weeks and post some minor update too, then possibly consider a redesign 2 years down the line at which point they are likely to want to throw out most of what they have anyway which doesn't matter since they spent maybe $1000 on the whole thing.
If the site provides any kind of complex functionality then wordpress is really no longer going to make sense as a core to build around. Here 90% of the site is likely to be customer forms, order processes etc that don't fit into any convenient pre-existing model. At this point if you build around an off the shelf CMS 90% of the site will be custom plugin code so there will not be a huge benefit to anyone who would take over the code.
It's all about making the business itself a first class citizen rather than a particular piece of software.
People dont want to learn a new system, and they are confortable with wordpress.
There are millions of devs and designers also familiar with wordpress - which can mean they can leave you in an instant and get anyone else to make changes or add feature to the site easily.