Honestly, it's a really simple issue to grasp.
Impeachment is a political process, not one that abides by ethical principles of justice. The only reason truth and decency matter, to an impeachment proceeding, is to the extent that it matters to each individual member of Congress.
It doesn't matter how incompetent Trump seems to be, how clearly it appears he's obstructed justice, how deep his connections to criminal underlings. What matters is if Republican members in the House and Senate will vote "yea" to impeach him. If they won't (and I'm comfortable saying they won't, on purely partisan grounds), then unless you've got enough votes to push it through in House and Senate regardless, impeachment is a waste of time and effort. The Dems might get the House, but not the Senate, the way things are right now. That's likely to change in 2020, but it may also no longer be an issue in 2020.