Possible. As others have noted, WalMart has done similar things.

I think a retail/warehouse workers' union only really works if it's a national sort with local chapters so that a company can't hire except through a union.

I'm not saying such a situation isn't rife with its own problems, but as long as these unions are essentially on a per-location basis, a company with the resources can always close one location and open up down the street to avoid the union.

Unions need several things to work: two of the big ones are a job such that you cannot replace someone easily; and a sense that "we are all in it together".

If you don't have the first it is easy to replace anyone striking, and the union doesn't have power. If you don't have the second the better employees will want more money and so they won't work with the lesser ones

In retail it is generally easy to replace people. It only takes a few minutes of training and the new person will figure it out.

In retail there are a lot of different jobs. People who can do more (and have been around longer) are worth more. Many of the jobs do have metrics that can be measured to say who is better. While the pay this results in isn't much more, it is enough.

That isn't to say you need the above to have a union. There are counter examples that work, but they are less powerful than unions that have those factors because they find it harder to get everyone to stand together.

As such retail unions have trouble working. It is too easy to hire someone else if they go on strike.

Yes. This is why unions have to be big to be effective. The union has to be big enough to cover anywhere the company might be able to move to. This is why the Teamsters work so well, you can't outsource US trucking to Namibia.

This is also why the UAW started dying once containerized shipping made it practical to move factories overseas.