So polestagger gets removed, and you just want to replace it with knockdown? How about we just buff the stats of those weapons instead, especially since this is already being worked on(or already done). No sense adding skill-less RNG to weapons that just lost their RNG gimmicks. More skill, more depth, less RNG.
2-3 speed buff and a 3-4 blunt damage buff, and I'd consider them well reimbursed for the loss of polestagger.
Edit: You really want to give knockdown to 150 length weapons with 95 or 97 speed? Knockdown, so they can hit you once with a swing, stab you on the ground, kick you as you're rising, then stab you again. This combo would almost certainly gib anyone. Terrible idea. The weapons need a buff, but this is just a terrible way of going about balance. They'd be longer and faster than any current weapon with knockdown.