I have to admit that this is a very good point, I think an API is something that can be difficult to change once it's written because by definition once it's out there other programs will rely on it staying the same. Getting it right the first time is better than trying to retain backwards compatibility with a badly designed sytem. Writing an API is a big responsibility, especially if it ever gets used with something as hugely significant as Wikipedia.
Well, I've been kind of thinking on the following lines: Each MediaWiki wiki administrator - in the case of Wikipedia that would be Wikimedia - can monitor what API calls are still regularly used. It's not really something like HTML where you want backwards compatibility because you don't know how many millions of unmaintained old documents still exist and still want to be read. We do. We can tell if the usage level of an old API function has dropped below a certain threshold, at which case we can decide to stop supporting it. Although we will end up requiring some tool developers to use the new API and some users of unmaintained tools to use other tools, we can be sure that it won't be many. Hence I don't think backwards compatibility is going to be a huge long-term requirement for us.
Timwi