The argument against the Democrats isn't that there be no witnesses, the argument is that witnesses who did not testify to the house should not testify in the Senate. The House felt that it had enough evidence to impeach when it voted, but now feels that it does not have enough evidence to convict, and is demanding that the Senate do investigation that was the responsibility of the House.
It looks like there will be witnesses though, according to McConnell - now the dems are screwed. Joe, Hunter, the whistleblower, Schiff, Nadler, Pelosi, every White House staffer who jumped ship to work for Schiff...... you begged for it - now you're gonna get it.