A whistleblower sounds the alarm and if it's deemed credible it is investigated. Trump's actual accusers are the witnesses and he can face them and testify himself. He can also allow others that can exonerate him to testify.
I believe you previously stated you believe Trump absolutely did attempt to exchange the release of Congress approved aid and an Oval Office visit for the release of the aid.
We are supposed to have checks and balances, oversight. It looks as though what the Republicans and Trump supporters really object to is anyone sounding the alarm if they see something that would indicate an abuse of power. It's wrong and it's dangerous.