Why won't trump let anyone testify? What's he afraid of? That's why he was impeached in the first place because he obstructed congress.
Are you saying, that when a president does not allow someone to testify before congress or with holds documents from congress, he has something he is afraid to reveal? And every time a president does so, it is an impeachable offense? (Think back now. How often did Obama do likewise?)
Perhaps Trump just did not want to join the charade, as he so stated, and lend any part of it a sense of legitimacy.
HOPE IS MOTIVATION; REALITY IS PROFIT