You're missing the point. The problematic aspect of Postel's Maxim is not "be conservative in what you send" but "be liberal in what you accept"--as the draft points out, being liberal in what is accepted tends to cause people to become liberal in what they send and prevents implementers from tightening what they accept. HTML is the best poster child for the detrimental effects this causes (HTML5 mandates how you have to parse tag soup, since any HTML parser that wants to work has to parse tag soup that way since it is very much used in practice), but HTTP and MIME are easily in similar boats.
The point of the draft is best summarized as "if you can detect that the other side has a problem in its implementation, raise red flags early and noticeably." It's not safe to recover to some default, because that can make you think that things are working when they're not--imagine if the engine control software defaulted to assuming a different type of engine than what existed. The resulting confusion could equally destroy the engines; this is similar to what happened to the Ariane 5 rocket that caused it to explode.
The point of the draft is best summarized as "if you can detect that the other side has a problem in its implementation, raise red flags early and noticeably." It's not safe to recover to some default, because that can make you think that things are working when they're not--imagine if the engine control software defaulted to assuming a different type of engine than what existed. The resulting confusion could equally destroy the engines; this is similar to what happened to the Ariane 5 rocket that caused it to explode.