Trump has only two choices when served with a grand jury subpoena. He can testify or take the fifth. No one is above the law.
That is a nice sentiment; however, you are going by emotion and your own sense of knowing what is fact and what is not. As pointed out, the 1973 DOJ Memorandums signed under Clinton established DOJ Policy, with consideration to the Constitution, that states a sitting President can NOT be forced to testify / appear before some Prosecutor's kangaroo court just because he wants a sitting President to do so. It also established that a sitting President can not be Indicted.
Bill Clinton was never Indicted - he was Impeached...by CONGRESS. Mueller and Democrats seek to force the President to testify before a proven biased political entity whose agenda, as shown, is to by-pass Congress and 'un-elect' the current President by entrapping him and Indicting him.
The same people who are investigating Trump and his team are the same ones who have already proven they can not be entrusted to carry out an un-biased 'interview' based on the fact that several Democrats perpetrated the same crimes of 'Lying To the FBI / Investigators' as did Flynn but through 'selective targeting' only Flynn was Indicted, proving the 'Investigation' has an agenda and an identified, pre-determined group of targets.
And again, finally, the lead Special Counsel has issued a threat to the President of the United States, threatening to 'put the President in Jail' if he refuses to comply with a potential subpoena to undergo an 'interrogation' regarding....a NON-CRIME. Mueller has not identified any specific crime that has been perpetrated; yet he wants to issue subpoenas and threats to a sitting President?! As one top lawyer put it, Prosecutors don't just get to subpoena a President because they 'demand' answers. The only reason a sitting President should be subpoenaed is to obtain information that can not be obtained any other way, information about specific criminal activity being investigated. Mueller has no specific criminal activity - he has no evidence of ANY criminal activity regarding non-existent illegal Russian collusion.
Without evidence of any crime Mueller's demand / threat of subpoena and 'jailing' the sitting President if he refuses to comply with said subpoena is more than enough evidence that Mueller is on a 'fishing expedition' and how badly he needs / wants the President in front of him answering his questions meant to entrap the President. It is clear that, having to crime already identified, Mueller seeks to CREATE one in his 'interview' with the President.
No evidence of a crime : no crime to be investigated. (Never was.)
No evidence of a crime - no justification for subpoenaing a sitting President.
Mueller is in the final phase of his Witch Hunt, and everything now depends on forcing Trump to testify under oath where he can possibly ensnare the President. Creating this crime is the only hope for finding a crime involving Trump now... and everyone knows it.