I don't believe European countries would actually dare to expel US troops. Any European politician criticizing Trump's actions could be seen as endorsement of the US Democratic Party, or the neoliberal establishment. However, if Trump were to take the crucial step of forcibly occupying Greenland then expelling US troops stationed in Europe, this action would be seen as a direct challenge to US national interests. This is unlikely to be accepted by the neoliberal establishment. In other words, Europeans might verbally criticize Trump, but they probably wouldn't have the guts to actually do it.
On the other hand, global military presence, including NATO, aligns more with the demands of the traditional establishment, which is not what Trump values. If European countries expel US troops, wouldn't that perfectly align with Trump's isolationist ideology? How could European politicians aligned with the neoliberal establishment possibly act in a way that suits Trump's thinking?
Therefore, whatever Trump does won't truly affect the existence of NATO.