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The question is not whether you need alternatives, but whether you want them. If you don't care about AF and IS (they have their utility but don't directly contribute to the aesthetic of images), but attach more importance to pure IQ, Leica may offer something superior to the Canon big whites. Douglas/telyt mentioned in a similar discussion that the Canon 400/2.8 disappointed him, compared to his Leica's.
There are not any real alternatives for those 400/2,8--600/4--800/5,6 lenses even if you skip the IS and AF
And you can't replace the 400/2,8 with the Leica 280mm f/4 APO as suggested in a couple of threads before