You can do this, because there's enough good plugins that it can be supported.
You're right of course that you cannot support 100% of all custom code ever written for WordPress. But it's a bit heavy-handed to conclude that therefore it cannot be done, don't you think?
most of the code written for wp i saw is actual spaghetti-code. including so popular and later blamed thesis theme. there is no enforced standards in coding, no clean versioning between modules and wp.
You're right of course that you cannot support 100% of all custom code ever written for WordPress. But it's a bit heavy-handed to conclude that therefore it cannot be done, don't you think?