None of the original developers are still at the company and one of your customers has discovered a bug in their implementation because of some obscure edge case and now you get to spend the next month understanding the complex code and writing a patch.
Writing maintainable code is not the same as dumbing everything down.
It is hard to understand. It is efficient. It probably requires computer science knowledge to solve the issue. And yet it is as maintainable and well documented as they possibly can make it.
Not everything can be 'business logic' in the sense of 'something a manager can understand but not code'.
Some algorithms are just hard. And they are worth it.
What would be Google without PageRank? What would be Skype without their LAN piercing modules? What would be iD without their game engines? What would be CryTek without their game engines?
None of the original developers are still at the company and one of your customers has discovered a bug in their implementation because of some obscure edge case and now you get to spend the next month understanding the complex code and writing a patch.
Writing maintainable code is not the same as dumbing everything down.