Never going to happen, there will always be a point where the company will try to minimize worker salaries as far as it can get away with.

>Wouldn't that corrupt union leadership over time?

No, you would pay employees using profits in part, and implicate them into the decision-making process. That way the incentives of union-leadership, workers, and stakeholders align. You can also push for union structures that minimize the power of union leadership and instead give it to the rank and file. Basically, making your company a bit more into a worker-coop fixes the issue.