> When is loyalty meaningfully rewarded?

At the risk of being overly obvious, a company might do things like increase your compensation (especially backward looking discretionary comp), give you more opportunities suited to your interests, or consider others for layoffs before you.

Of course, none of these things are guarantees, and it's entirely rational to assume they will never happen and go about your professional life accordingly. But that doesn't mean they don't happen at some companies for some people.