Based on my experience, people sit on the sidelines because they're comfortable. People advocating for change should be polite first, and then not polite if politeness fails.

So you plan to force me into accepting your position if I don't take it when you ask nicely? I'd say you're already not polite.

You already exist in a democratic system (assuming you're posting from the first world) where majority (mostly, some caveats) rules. This isn't any different. Everyone gets a vote, not everyone gets their way.

Not quite. I live in a republic. The majority wins elections but still don't get to totally trample on the minority, because we have a limited government.

Now, a union election isn't a government election, so that doesn't totally apply. But, for example, the majority has decided to pass legislation that makes my current state a right-to-work state, so a union can't require me to be a member, even if they win the election.

Of course, unions could campaign to change that law. But that's a different set of elections they'd have to win...

Is it at all possible, in any scenario, that people might have other reasons than purely personal comfort for sitting out a particular push for change?

It is possible, and I am receptive to enumeration of those reasons. One must be able to entertain both sides of an argument. Selfishly, it would help me strengthen my own argument.