The question is context. Simply asking the foreign power to investigate corruption and moving classified information to a secured server isn't illegal, regardless of the optics. Context is what crosses into the legality of the issue and the whistleblower's motivation and potential contamination changes some of this context. It also provides necessary detail to actually prove the President committed a crime.
The only reason you would not bring the person forth is if you knew that doing so would harm your case. So this makes it a suspicious claim and brings doubt into the process.
Of course, Trump brings doubt into the process as well because of the shenanigans on moving data to a classified server in the first place, as well as asking a foreign power to investigate corruption that just happens to be a person running against him in the next election.
The whole thing stinks and the only way to get to the bottom of it is complete and total transparency.
I do agree that the whistleblower will need to come forward, but he/she shouldn't have to come forward until legally required to. I'm not familiar enough with impeachment but I'd imagine if we go by our judicial system, that's not until the trial, meaning when it gets to the Senate.
Our justice system is setup that way, likely following Salem With hunts, so yes, the accused should face his accuser, but Trump, nor his allies at Fox or the Senate get to dictate when. Not having an accuser come forward sets precedent for pretty much every president getting false accusations lobbed at them.
Let's not turn a blind eye to Trump and his cult attacking people who turn on him, he's set his bed and now he has to lie in it.
The American public just the same has a right to have unredacted materials released, meaning transcripts and recordings (if they exist), whether it's just this call or others may be up in the air, but let's not be selective about Trump facing his accusers, while his backers champion every obstruction by him to discovery attempts by the House.
You think Trump is going to assassinate this person?
Please. I am fully in line with a total investigation and then impeaching Trump if guilty of wrong doing. But you can't just rely on anonymous testimony with no proof or cross examination.
If the whistle blower had hard evidence, like "the murder weapon can be found here" and then prosecutors go and find the murder weapon, sure, the person can remain anonymous. That's the whole idea behind programs like "crime stoppers". But if the whistle blower is using testimony like "Trump said x, y or z" then the whistle blower has to make an appearance and offer testimony so that the witness can be determined to be credible through cross examination!
Having said the above, I really don't see what else the whistle blower can say that hasn't been already corroborated by the WH memo; there is nothing else to gain. I suppose he/she could try to interpret their feeling of the context of the conversation, but I don't even see how that would be admissible.
At this point, outing the whistle blower only helps politically, not judicially, as painting him as some partisan "deep state" operative could sway opinions.