Subject: Re: Audio of Trump's Conversation
Then congressional subpoenas are useless if compliance is at the whim of the person served.

Not at all. You can be convicted for contempt of Congress if you fail to comply with a Congressional subpoena. That's what Bannon was convicted for - he refused to show up for his subpoena date.

What a Congressional subpoena doesn't do, though, is impose an obligation to retain documents you might otherwise delete in the ordinary course of business. That makes it different than judicial proceedings and law enforcement investigations, which create an affirmative obligation to preserve documents and materials that might be evidence in those proceedings. Congress is engaged in a legislative endeavor, not a judicial or law enforcement one that is bound by the rules of evidence.