Bad idea - you don't know what namespaces you want to use in future.
So? I rather suspect the number of desired namespaces is a lot more limited than the number of entries that could use colons, and I doubt that there would be any overlap, especially since we're free to engineer nonsense-named namespaces.
It's an imperfect idea, but making colons reserved is just as bad, if not worse, than making slashes reserved. There are more times a colon would be used in a title than a slash.
I.e. making colons reserved is a bad idea too. We should consider the thought of unreserving them.
For example, one thought is that only in the case of namespaces would you find the match to [a-z]+:\w ("wikipedia:FAQ")
since normal English use of colons only matches
\w: \w or \d:\d ("Alan Turing: The Enigma" or "4:25")
Are there any counter examples?