Arthur Scargill is currently gets an expensive house in London paid for by the dwindling numbers of the mine workers union so there is no doubt some truth in what you say.
On the other hand the unions are the only real prospect of wage negotiation available to the majority of employees so regardless of their faults they provide a necessary role that doesn't otherwise exist. Sure its possible to leave your job if you don't like your salary, but there are many reasons you might not want to, such as commute distance. I see employers as price-setters and employees mostly as price takers which pretty much means the wage will tend to be on the low side.