Yes, those are two obstacles that employers face when hiring strikebreakers, along with more significant obstacles like laws forbidding strikebreakers, licensing, and union violence.
The common factor is that it's not enough to refuse to work, unions must be able to ensure that no one does the work.
> unions must be able to ensure that no one does the work.
Often the barrier is natural, such as with folks at Google who built the systems and are (collectively) the only ones who can keep them running without significant interruption. By what authority, for what compelling reason, would you abridge their freedom of association (or in this case non-association)?
I wouldn't, they should be free to work or not as they choose. But they shouldn't be able to abridge anyone else's freedom and I doubt they're as irreplaceable as you think.
Doubt all you like. It doesn't matter. What matters is whether Google doubts it, and I think they know better. They know the costs of hiring and training and downtime for their business. Do you seriously suppose that they think they could just find a few thousand replacements on a moment's notice?
Now you're making a different argument. Yes, there are costs involved in replacing employees, and there are costs involved in allowing employees to walk out or refuse to work on certain projects. Naturally Google will weigh those costs.
But earlier you claimed they were "the only ones" capable, and that I wanted to abridge their freedom of association. I disagreed with both those statements.
Now that you've restricted your position to acknowledging a reasonable cost/benefit tradeoff for Google, I agree.
> earlier you claimed they were "the only ones" capable
No, I claimed that they were the only ones capable of running the systems without significant interruption. Others can be trained up, but it would take months and there would be some failures along the way. You're the one whose argument has been changing, since you started with the presumption that strikes' effectiveness relies on explicitly excluding others (e.g. via law or force).
The common factor is that it's not enough to refuse to work, unions must be able to ensure that no one does the work.