I think you misunderstand their “plea for witnesses”. They don’t even have any witnesses and they’re pleading for a witness to come back ahead. Schiff’s talk of more e mails coming out, more witnesses coming ahead, more facts being offered is just wishful questioning on his part – he’s completely convinced that Trump’s a crook and therefore there has to be proof available someplace that proves it. It’s no alternative than me promising I can 100% guarantee you the 49ers are going to win the Superbowl. No I can’t, but that’s unlikely to forestall me from claiming “see, I told you so” if they do wind up successful.