You pretty much answered the question yourself. If you know they're going to switch block stance from high to low in rapid succession, then you know how predictably they are going to block, right? Mixups and stagger pressure. No one is going to sit in a corner and just alternate guard all day. If you're really that worried about it, watch any high level match videos, and I guarantee you, alternate guarding is very minimal. It's strong on paper, but you're overestimating its usefulness in the overall flow of combat.