Sooo...how do you measure how good a programmer is? You've got to cut $200,000 from your budget. You can lay off 2 junior programmers, or 1 senior programmer.
How, exactly should management determine whether or not the senior programmer is worth twice what a junior programmer is?
This is not a rhetorical question, and generalities like "keeps the juniors unblocked" are not an answer, unless you can give some objective metric which really tracks that. And even if he "keeps the juniors unblocked"--how can a manager tell whether keeping the juniors unblocked is worth $200,000 a year?
Management doesn't agonize for months of who stays and who goes. The directive comes down from Olympus and they have to be executed quickly so that P.R. can move on to another narrative.