From an individual senior exec point of view - all staff are replaceable. You just hire from outside the company.
People don't think in terms of shared commons and that if all companies are doing the same thing then there won't be much of a "senior" market left to hire.
When companies stop hiring juniors it completely kills the employee leveling process. It all but stops the creation of skilled senior employees.
You say you can just hire from outside the company - but what do you do when there is no one left to hire because the talent pool is completely drained?
Abandoning the junior employee will slowly drain that talent pool until there are no seniors available to hire, the "just hire from outside the company" plan doesn't work any more.
Because those senior people will NOT be around forever. And they have killed their talent development and knowledge transfer pipelines.
Either direction you take it, this feels like a lose-lose situation for everyone.