Very true, but the allocations remain fixed. Their value doesn't really change based on individual performance, which is part of why they see them as "lost" when they walk away. On average though, they tend to at least track inflation. In practice, you'll find their effectiveness for retention trends to track a company's prospects. If you work at a company whose stock has done well over you've been there, you could see why it'd be hard to go somewhere else and be as well compensated as you'd be staying where you are.
RSU values will also go down if shares prices go down. In practice they are much more volatile than salary, people's RSUs fall in value by >50% all the time, but it's pretty rare for people to get a >50% cut in salary.