Arnold's movies bomb because they're derivative, by-the-numbers, generic, bland, predictable, totally forgettable and he's a wrinkly old man who still thinks it's 1992. In rehashing the same crap over and over again, he exemplifies what is wrong with Hollywood.
Hollywood has one job; entertain us. People aren't boycotting Schwarzenegger films because he says mean things about President Trump. They're boycotting his films because they're not very good. Twenty five years ago, the crap he makes today would've gone straight to the video rental market and not even bothered with a theatrical release.
They are boycotting Arnold himself and the movies. He wants to be never trump. But boycotts work both ways. Best for businesses to keep their mouths shut and work on making money, rather than enemies of customers by boycotting.