Note
Defining success
Major goals need measurable key results, even when the measure is only a proxy.
Major goals need an explicit definition of success. The objective gives direction, but the key results say whether the work is moving the right way.
That is the basic logic of OKRs, and it still works when the key results are only proxies. A proxy can be useful if it makes progress visible and keeps the goal from staying vague.
What matters is having something concrete to measure against. I care more about being able to track progress honestly than about finding a perfect metric. I want to decide those measures in the same planning pass where Planning and implementation happens.