Making employees replaceable cogs is what industrialization was completely about. It's what happened during globalization. Think about all those seniors who lost their jobs when the factory went overseas. That was successful in large part because the distinction between a junior and a senior is not that great.
There would be some exceptions here for management and execs, but we are not talking about them here.
No it wouldn't, because a senior worker wouldn't be around to say things like "oh yes we use a jig under this circumstance that we keep over here <points>". Every business has ton of institutional knowledge like that.
To be clear, I'm not actually disagreeing with your point, I do think it's important to have those people. But companies felt otherwise.