This is true, but you are counting on people who stake and sell their votes to be rational about that. Sometimes humans are stupid, especially when incentivized by greed. They'll just see the short term incentives (money for vote-selling) and not consider they might not be able to get out in time. They will think, "I will just sell my keys before everyone else and get out quickly after getting nice interests for a while".
Again, it's a reason we need another way to incentivize skin in the game voting, in addition to lockups.😉