There is a flip side to this: why should you have the ability to take the hard work of whomever came up with Mickey Mouse and take it in a direction that the creator doesn't like?

Say you wrote an open source software package in 2004. Should Microsoft be able to take your work and incorporate it into a proprietary package, just because 7 years passed?

Yes, because even the founders recognized that intellectual property is not real property. You can't stop people from copying ideas, and so they permitted copyright to be applied for a limited time in order to encourage people to release more creations (because they got rights to exploit them exclusively for a while), because the idea was that these creations entering the public domain would enrich the public domain for everyone. The fact that nothing is falling into the public domain (and probably never will again in our lifetimes if the status quo continues) is the biggest flaw in the current copyright system.

No, you don't get to own an idea forever just because you thought of it first. The very concept of 'owning ideas' is silly, but even if it wasn't, the second you tell it to someone else, it becomes part of the collective society and culture. If you don't like the deal that copyright gives you, you have a choice: don't share the idea at all. And that's fine, and I'm sure that the world would have kept spinning if nobody ever drew up MickeyMouse.

I don't think that's the flipside to my point. I think the flipside to your point would be "why have IP protection at all? It's an idea in my head now, why should someone else have control over that and my ability to execute it?"

And I don't actually program anymore, so let's go with writing: should anyone have the ability to republish it without my consent after the copyright has ended? Yes. Even a big publisher.

Should anyone have the ability to take my created work and extend on it, using my creations outside of the created work? Yes, after the trademark ends.

On the other hand, there is the problem of abandonware that is so old the owner don't actually enforces copyright on it anymore but technically it is still copyrighted.

I don't know if that's a "problem", it's just piracy. I think the abandonware scene is what film would look like with shorter copyright. Well, and non-traditional sequels and remakes may have actual budgets...

Well that depends on your flavor of open-source, doesn't it? Didn't Microsoft use some BSD code for their TCP/IP stack once? (I'm sure it's long been rewritten.)