Under the Wagner Act you can't be fired for concerted collective action in the workplace, if the goal of such action is to improve working conditions. That is a much broader protection than for just strikes (which are pretty narrowly defined).
But like you say, any corporation worth its salt can find ways to get rid of problem employees without violating the letter of the law even when it's not already planning big layoffs.
that's likely a major motive behind RTO, yes - voluntary attrition and termination with cause (ie no severance, unlike layoffs and pip). A large walk out would probably boost share pricing.
