When a union perceives that the company is not negotiating rationally, that's when they take a vote to authorize a strike. Then, hopefully, the strike doesn't happen... but it's available.
Seems that if the employer can hire replacements quickly enough, the strikers don't get their jobs back.
"Economic strikers defined. If the object of a strike is to obtain from the employer some economic concession such as higher wages, shorter hours, or better working conditions, the striking employees are called economic strikers. They retain their status as employees and cannot be discharged, but they can be replaced by their employer. If the employer has hired bona fide permanent replacements who are filling the jobs of the economic strikers when the strikers apply unconditionally to go back to work, the strikers are not entitled to reinstatement at that time. However, if the strikers do not obtain regular and substantially equivalent employment, they are entitled to be recalled to jobs for which they are qualified when openings in such jobs occur if they, or their bargaining representative, have made an unconditional request for their reinstatement."
If you want to ask, "is it generally legal to fire all members of a union in retaliation", the answer is "it's complicated". Read
I'm going to guess you don't know any union members.
Strikes are very hard on the strikers. You're suddenly taking no income but still putting in a shift on the picket line. Not only is there the real risk of the whole thing being net-negative, but you could lose your job entirely.
And a lot of time, you only strike because you've been squeezed financially for so long, you may not have much of a buffer for all that.
You make it sound like unions strike for sport. I suspect a conversation with an actual striking union member might provide a different perspective.
And yes, as I explained but will repeat, strikes happen within a political/legal/cultural/economic context. In some contexts, that has increased the frequency of strikes. By your account, the proportion of strikes just relates to the financial position of the strikers...I don't know how it is possible to be aware of the history of trade unionism and come to that conclusion.
For example, individuals do not choose to strike. They vote for strike action in a ballot or there is a decision taken by an executive of the union. Exactly how this occurs has had a huge role in determining the frequency of strikes (if you look at the labour history of the UK, the lack of democratic process in important unions was a major reason why they went militant, why strikes increased, and eventually why unions fell into decline).
The specifics of your account of striking is mostly wrong too (for example, it is not always true that strikers are unpaid). Most people today do not understand how unions operate...this is related to the fact that they live in countries where the labour movement committed suicide, and they are left with militant unions that misrepresent how unions can actually work effectively (again, Europe is a perfect example of this, the militant behaviour of unions today is a function of their irrelevance in society, and that is 100% a function of the use of industrial action by militant unions).
If management thinks the union is too weak to strike, they can stonewall. And if the union can't muster a strike, management tends to win. If the union CAN muster a strike, then (1) people in the union develop a stronger belief in their union and (2) management updates their estimate of what they can get away with.
If a union isn't threatening to strike it's because whatever they expect to gain in leverage and negotiating power isn't enough to offset the cost of the actual strike.