You're absolutely right -- it can definitely become difficult to achieve that ideal with certain clients, and as you've also said, "simple" and "maintainable" can be a very subjective thing. I admit that it's tacit knowledge in my case, and also a gamble that the next developer will preserve their sanity. There's no formal yet practical way of gauging simplicity and maintainability.

Your point on mental comfort is also spot on -- ideally, there should be a satisfaction between developer comfort and fulfilled clients. Sadly, that could also come with its own compromises. Heh, development is fundamentally a game of figuring out the sacrifices with the least impact for all parties involed.