Refugees in Germany probably create a situation in which base security is tightened. An expensive hassle. For Germany not to pay up after creating the problem is pretty much bullshit.
US Probably has to consolidate around K-Town-Ramstein-Landstuhl, but K-Town is pretty hard to secure.
Germany, itself, is pretty much defenseless if US pulls troops. Have to plan for sleeper cells among the migrants there. It will cost a lot more for them to build up than it would be to pay their NATO share, I'd think.