I believe the parent is saying that if your logic were correct unions would be extremely powerful in relation to multinational corporations and everyone would be afraid of getting the union angry, as this is not the case something must be keeping unions in check or they cannot, at any rate, grow to be as powerful as you suppose.
That your logic as stated does not hold any identifiable logical error does not mean that it gives the correct result.
And unions are exactly that powerful. A strike can easily shut down a company, a town or a country. I do not see a similar power on the corporation's side. Firing everybody would just kill the corporation as well.
Now it's true I have not lived all over the world, but I have lived in numerous places in the U.S, in Denmark, and spend some time in Italy every now and then. I also lived in Germany as a kid, but I've never really seen these fearsome unions you are familiar with, and did I say again I am in Denmark? Even here it isn't like such a powerful organization.
Certainly one can find historical instances of powerful unions, but they do not seem to exist now, and thus your analysis must be faulty somehow. If unions are naturally more powerful than the other forces of society their power should always increase and not decrease, but as the data linked above shows the opposite is the case.
If you want people to accept your logic you will probably need to back it up with data, arguing about real world things like unions tend to bring forth such requirements.
In Europe I've seen airports and cities and even countries dead for weeks due to strikes. In my country I've seen union leaders become politicians and then in jail for corruption. In US I've seen evil laws (AB5) imposed on people by politicians (Lorena Gonzalez) owned by unions (Teamsters).
But I am open to the possibility of mistakes in my logic and dfxm12 post above had some good points.
That your logic as stated does not hold any identifiable logical error does not mean that it gives the correct result.