Given they never dreamed or seriously considered they'd get 2/3 in the senate, it's then useful to ask what the real motivations were. What were they actually trying to accomplish? I think a motive that has to be considered is that they are actually quite confused, and do not have clear motives.
I think the House was thinking more about the case they were presenting to the country than the Senate.
Also, they put a lot of republican senators in swing states in an undesirable situation by making them actually pick a side on something they have always defaulted to political non-answers when asked about. It's pretty common in congress to propose and force a vote on something that you know doesn't have a chance of passing just to get a record of it. (The Republicans did that often during the House Impeachment hearings, and the Democrats are doing it right now with amendment propositions, for example)
All in all, I'm not comprehending what the witnesses were intended to accomplish and for what goal.
The whole trial was on whether or not Trump did something bad, and how bad it was.
During the House hearing, Bolton said he would challenge any subpoena in court, which could take over a year.
Before the Senate trial Bolton said he would not fight a subpoena, and that he had information that had not yet been made public.
'all hearsay, no direct evidence' had been repeated over and over in defense of the President. Bolton would be able to provide direct evidence.
By refusing to hear any witnesses, and the fact that Pompeo, Mulvaney and Bolton were all considered 'democrat witnesses', is evidence that there is a cover up happening.
The House has every opportunity to present all the witnesses and evidence it wanted. Now that they have zero authority in the situation because The Senate is in control of this portion of the process, they feel they can dictate terms still, in spite of that being the whole intent of splitting the authority between The House and The Senate.
If the Democrats called them, then yes, they are officially the Democrat's witnesses. Gold star for trying. Why is it so important the subpoena is enforced in under a year? Oh right, it is an election year. Not transparent at all. Can't win at the ballot box so you have to try to win using the idiot box.