Seniority is a terrible system to build a foundation from.

There's nothing inherent in union membership that forces seniority to be a requirement. It made a certain amount of sense in commodity trades, but the volatility of skill requirements in software and high tech invalidate that assumption.

Unions can add a lot of extra process to things that don’t require it

That can happen. And then there are the things that do require it. Better to have dismissal for a legitimate cause than at-will employment and an arbitrary kick out the door.