I don't think that's the whole story, for two reasons. Firstly, although of course getting higher rates of compensation is an important part of the role of a union, and they're very successful at doing that, unions are not just there to get higher pay. There are numerous other ways that individual employees can find themselves on the wrong end of bad employment practices, unfair management, harassment or other types of discrimination that aren't picked up by a company's internal processes. A good recent example in the news was the housekeeper that was allegedly assaulted by Dominique Strauss Kahn at the hotel she worked at in New York. No fault on the part of the employer of course, and I'm sure Sofitel pay isn't bad either. But, in a situation where a high profile and important client has allegations made against him, many hotel managers would be tempted to sweep it under the carpet, or worse, quietly let the employee go if she caused too much trouble, for some "other" reason of course. But because the housekeeper in question was a member of a union, the allegations had to be taken seriously, and she would have had access support from someone with expertise if there were any difficulty. The lesson there is, if you're going to assault a maid, don't try it on in a unionised hotel. Of course it would be better if all workers had that kind of support to help them.
Secondly, Apple Stores can pay higher wages than others because they're a successful business with high margins, and the actual stores themselves form part of that. That's great! Unions always like to see businesses being successful and generating income. It's not true to say that unions want business to be less successful or damage their business. But, it could be possible to query how the profit is shared out. Seems to me that whenever a company is making some big margins, and if it possible more of that is going back to the capital owners than to the employees, then that isn't as fair as it could be. I think from what you're saying, that you believe somone lucky enough to be working for a company that's doing well, should mean they get paid better than a business that doesn't do so well. Well that seems only fair. But there are definitely cases where employees working for successful, profitable companies aren't getting compensated in proportion to the success of their business, especially lower ranked employees. So I believe there is always a role for proper representation, if only to put that case.