As I pointed out, Trump can just use executive privilege as cover for his fifth amendment rights & nobody can do a damned thing about it but Congress. You won't find 67 votes in the Senate. I doubt that our SCOTUS would rule against Trump, anyway.
There's still a whole lot of investigative work & unfolding going on in the background we can't see. They're just starting in on Michael Cohen. I doubt Trump had really good opsec along the way so there are lots of minor players in Trump doings we don't know about, either. Hope Hicks was recently spotted boarding AF1. Dunno what that means, but it probably means something.
Nixon's impeachment didn't hinge on his refusal to turn over the tapes to Jaworski per the SCOTUS, anyway, but on his refusal to obey a Congressional subpoena to do so.
I don't think that you could refuse a subpoena claiming executive privilege broadly. You could claim privilege for certain questions, but at least much of what Mueller is interested in is candidate Trump not President Trump so privilege wouldn't be cover, and his only valid refusal to answer would be fifth amendment rights. Also, since Trump allowed Comey to testify about his meetings, I think his opportunity to assert executive privilege if he were asked about them has already past. Now, he could still illegally refuse without asserting fifth amendment rights and we get to the question of what to do about it. That alone is grounds for impeachment, but it depends on Congress to act. Even if they don't do so for this offense, if the conclusion of the Mueller probe leads to impeachment, his refusal to testify is pretty solid evidence against him then.
But it's also not 100% certain that Trump couldn't be criminally charged with contempt. Yes, precedent is not to charge a sitting President, and the DOJ has stated in their review that you can't, but technically it's not explicitly forbidden in the Constitution, and SCOTUS has never had a chance to weigh in on it. So it could happen even though I think SCOTUS would protect Trump in the end. If so, it would put more pressure on Congress.
Anyway, trying to compel Trump to testify at this time does throw is down some sort of crisis path, so I don't think Mueller would actually subpoena him unless he's at the end of his investigation otherwise.