Its absolutly untrue that sitting our support Trump. By definition it supports no one. It doesn't change the numbe of votes either candidate needs to win, it simply doesn't add anything to either. This is quite literally mathematically true.
Also there is always a default. Default is what happens if you do nothing or change nothing, which means by necessity default is not voting. Thats how defining a default works. In fact, US citizens arent even registered to vote by default, you have to do it yourself. Thats why people have been pushing automatic voter registration.
Im used to idiots, I'll be fine. Not voting doesn't magicaly cause either candidate to need more votes. Thats simply not how math works, it just doesnt add anything. If you have to walk 10 miles and I decide not to carry you one mile, you don't end up having to walk 11 miles.