Krug had 19 points at even strength through 51 games last season. That's a pace for 30 even strength points. Walman had 2 points in 24 NHL games last year, which is a pace for 6 points (7 if you round up). Krug was tied for 31st in the league in even strength points last season. Walman was tied for 227th. Your narrative that Krug does nothing but the PP isn't even remotely close to reality.
Walman has done absolutely nothing to make Krug expendable. Walman hasn't excelled at anything that Krug is paid to do and has never even hit 30 points in the AHL. Walman might have done enough to stick into an NHL lineup long term. That doesn't mean he has done anything to replace a player who plays a completely different role.
"He is overpaid for what he brings to the table" isn't even remotely the same discussion as "this prospect with zero pro experience and this 25 year old who has looked adequate through 25 games of NHL experience can replace what Krug does on-ice." Your hatred of Krug's contract has fully clouded your ability to judge what a player brings to the table on-ice if you believe that a (indisputably) top 25 NHL offensive D man is made expendable by guys who are genuine question marks to make the opening night roster.
Assuming you believe that he is worth somewhere near $4.5M, then you absolutely should recognize that a player worth that amount isn't on the verge of being replaced by guys who may or may not be 6th or 7th D men at the NHL level.