Someone needs a tissue.
Talk about clouding one's view with emotional ties to a player!
The basic, and obvious, flaw in your argument, is that you are basing your stance 100% based on offensive performance while discussing the most important defensive position on a baseball team ( excluding pitcher, of course).
To say Castro st 21,easily trumps Russell at 21 simply underlines the fact that you simply do not understand the SS position, and the impact it has on a team's D. Russell is vastly superior on that side of the ball, and that is by far the most important aspect of the position.
Ozzie Smith was a slam dunk hall of famer with a career total of less than 2000 hits. Castro will easily eclipse his total. Comparable value? Hardly. And, no, I am not criticizing Castro for not being Ozzie.
Andrelton Simmons will not lead the league in hits as Castro did. But you won't pry him away from Atlanta with a gun.
You need strong D at SS. We have been at a disadvantage defensively at SS for decades. We now have a guy ( or two) who can change that, AND are expected to add offense. And you are quite seriously - and quite emotionally- arguing to keep a SS whose ONLY asset was his bat. Once. Before he went on a steady and pronounced decline.
Can he bounce back offensively? Yes. Is it a sure thing? No. IF he bounces back, and miraculously adds 70 points to his batting aversge, is he a positive? No. He still would not be. He was negative WAR when batting way higher than he is today. I don't believe he is a .230 hitter. I certainly expect he will improve that. But it would not make him valuable. He currently can't hit anything but ground balls, is a DP machine (offensively only) and on top of all that, is completely lost mentally on a baseball field.
He runs the bases lkke a kid in training league. No knowledge of the right way. No awareness on the field.
Is Russell a sure hall of famer? No. Does he clearly have defensive skills, range, awareness, quickness? Yes. IF Russell never improves on a ,230 BA, the Cubs would look for upgrades eventually. But nobody expects he will. But if he does, they will change again. But he would still have considerably more value than a below aversge Defensive SS.
That is how baseball works. I am sincerely sorry to see ghat you do not understand that, and won't any time soon. You are inexplicably emotionally invested in a specific person.