Yet no one who was actually involved in this saw anything improper. Not the CIA people, not the Ambassador, not Trump's envoys, not the Ukraine PM.
The claim apparently is that someone who was there, the assumption being they are CIA, told this other person about the call. That seems to qualify as a firing offense right there, so it is also important to find out who the leaker was.
Another possibility is that no one leaked the call contents. Rather, this whistleblower received them from an outside party, eg Chinese intelligence. The mere possibility of that is alarming enough to warrant an intensive investigation.
Buddy, your missing one small detail, Trumps own inspector general determined the complaint to be urgent and credible. What are you arguing?