It’s high now because there were huge expectations that Trump would be successful at dumping Obamacare, passing a budget, making business friendly laws, and so on. However, as you can see, he hasn’t delivered a darned thing, and the chances of doing so get smaller every day. He has no political clout and no one trusts anything he says.
I would say that the stock market is pretty much ignoring Trump now. They are comfortable with the current status where he can’t do too much damage. Things may not be super favorable for business, but they will be stable for now.
So if impeachment takes place, you will see a slight drop due to instability, but then as Pence (who is no angel, but at least he is rational) takes over, things will stabilize and pretty much stay at a good level.
Pence will have the same problem that Trump is having – the nutty Tea Party (Freedom Caucus) that won’t let him pass anything. So the Democrats will be needed to keep things on an even keel.
So, barring nuclear war, things will be just about where they are, even if there is impeachment.