The obvious answer to 'how's that?' follows thusly:
If the designer designed the organism it (the designer) must have either a: designed it to become extinct when the environment changed or 2: did not for see the environment changing and the organism becoming extinct.
We know things become extinct. So a designer must be continual designing new organisms to replace those that have become extinct.
Therefore the logical conclusion is that the designer is rubbish at designing things (apart from living fossils [score one for the designer]).
That fact that organism go extinct proves beyond all reasonable doubt that if there is a designer he is rubbish at design.
Rather than adding a designer into nature Occam's razor would suggest that a rubbish designer is an extraneous variable.