Here’s an idea for a metric: average discounted stability.
It would be calculated as sum(R_i * S_i) / n, where R_i is retrievability of the ith card, S_i is the stability of the ith card, and n is the number of cards.
The difference between this and simply average S is that average S doesn’t take into account the fact that you won’t be able to recall 100% of your cards, only some fraction <100%.
While this would be a nice metric in the mathematical sense, it’s a lot less intuitive than “the number of cards that you are expected to remember” aka estimated total knowledge. As I said in my comment above, it’s not that we can’t come up with other metrics, it’s that estimated total knowledge is the most intuitive one.