Not sure where "tying pay to tenure" came up, unless that's how you view what unions do? A union just means a group of employees negotiating as a unit, what changes they ask for isn't dictated in advance.
This is how basically every union workplace in the US works. The pay-scale becomes very rigid. Why would tech be different?
How do you address, other than dismissal, the existence of the SAG and other unions for industries with vastly varying pay levels?
Well, in that the vast majority of union jobs are either public sector blue-collar, or both, sure.
Non-blue-collar, private-sector union jobs don't work that way, though they often have firm minimums (like SAG-AFTRA “scale” pay.)