So you're advocating for a union that pays a premium for those employees who have seniority but whose skills are no longer in much demand? At the cost of salary

So you're advocating for a union that pays a premium for those employees who have seniority but whose skills are no longer in much demand? At the cost of salary for those whose skills have remained in demand or who have kept up to date with trends? After all, otherwise what advantage does a a union have in your scenario? I doubt many well performing SEs would be happy to have coworkers like that in the future. The ones worried about future marketability have many options including saving a lager percentage of income, investing money in passive income sources, moving into management, etc.