Congress didn't just get the right to subpoena people though. You're correct Trump is making those arguments but the house still has the right to subpoena and a duty of oversight. I get where the delaying tactic might be preferable for Trump but it's not really a great legal argument he's making.
You apparently didn't watch the Mark Levin program I linked above. He
cited the written statements, and cited the written law, and the written past precedents in the impeachment proceedings of Andrew Johnson, Richard Nixon and Bill Clinton.
Past impeachment procedure has
always been for the 15-member Judiciary Committee to vote for going forward with impeachment, and then impeachment to be voted on by the full 435 members of the House.
That is
not what is occurring now.
Rep. Adam Schiff as judiciary chair, is locking up all authority to investigate in the Judiciary committee.
And not even the entire 15-member judiciary committee,
he is only allowing the Democrat members to participate. Yesterday, Rep. Matt Gaetz (R-FL), a judiciart committe member,
attempted to just sit in the room and listen to witness proceedings, AND WAS FORCED TO LEAVE! So there is not equal access to facts and witness deposition, Republicans are working blind, with no information.
Unlike the previous impeachments, the Democrats have set up the rules so only they can call witnesses, only they can ask questions,
all BEHIND CLOSED DOORS so they can completely control the information, and they leak out whatever gives the appearance of supporting their case against Trump.
Trump and the Republicans are not allowed to cross-examine witnesses, not allowed to ask questions or present any exculpatory evidence that would exonerate President Trump or his officials. It's a Soviet-style kangaroo court.
In three past impeachments, as Mark Levin cites and quotes, the ruling party bent over backward to present a joint bipartisan impeachment effort, where both sides had the same access to subpoena power and witnesses.
That is NOT what is occurring now.
