Once you admit there’s no public evidence and may never be, it’s hard to follow your logic asserting we’re heading for a Constitutional crisis. You assert Trump will “probably” take action to interfere with Mueller’s investigation. Why would he do that if there’s no public evidence against him?
Actually the Mueller investigation turns out to be a huge trap for the Democrats. Beyond obscuring whatever message they might have on healthcare and taxes, issues that people care about, beyond rallying Trump’s base who do see the lack of evidence as vindication proving the investigation is a witch-hunt, the investigation is dragging in Democratic Party victims. It has already played out that way. The Manafort indictment put a spotlight on Company B, and that turns out to be the Podesta Group, one of the biggest Democratic Party donor and lobbying operations, and that forces Tony Podesta to resign. Having gone all-in on criminalizing contact with Russia, the anti-Trump factions are waking up to the fact that they’re going to be haunted by their own history of Russian contacts. How many Democrats took money from the pro-Russian Ukrainians Tony was lobbying for? Does it hurt Trump to keep this going a while longer?
The other point is that there are other ways to sideline Mueller without anything as overt as firing him. Perhaps he directs Rod Rosenstein to expand the mandate of the Special Prosecutor to cover Russian influence going back to the Uranium One Deal and gag orders on FBI informants or anything that intersects the time when Mueller was FBI chief. Mueller would have to recuse himself.
In any event, it really doesn’t make a lot of sense to say there’s no public evidence but we’re still headed for a Constitutional crisis, especially when you don’t show that Trump has any deeper agenda for wanting to bring one on.