(Author here) I agree that the code is horrible :-) that's kind of the point - if you choose to hack up your code, then you end up with horrors like if(foo is Bar) { ... } else { ... }.

The type declarations are, I'd argue, not horrible at the outset, but once you introduce the new requirements then it becomes incorrect. I even suggest possible solutions including adding depth to the hierarchy - the point is doing that means yak shaving, not doing it results in a definitely horrible object-orientated design because it no longer fits the problem.

You can argue that you should have a better model from the outset given the coarse change which comes into play, which is potentially where my example starts to break down (I think it would be hard to find an example that would not break down in some way here, given the need to compress reality into a blog post), so take it as read that in reality the changes are often a lot more subtle than the example I give.

The point is that you're having to make far-reaching, rigid decisions up front. I have encountered this over and over again.

I am happy to admit it's an inadequacy in object orientated design on my part, and if somebody were to suggest approaches that helps mitigate this problem I'd be very happy, but I do wonder whether it's an inherent property of the whole approach.