How should we evaluate tools for thought? There's no simple metric, as far as I can tell. The best tools change your paradigm anyway, so your old metrics (books printed per year?) aren't what matter.
Here's one (vague, but focusing): how much meaning is unlocked on the margin?
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What would be the meaning unlocked on the margin for spaced repetition systems?
Something like insights when looking at old cards/cards mixed together? Or something like knowledge compounding?
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The metric that matters for SRS is not anything about memory or the cards themselves, but the marginal contribution to some purpose with intrinsic meaning for you. That might be powerful insights in one's original research or whatever; it's probably not what happens "in" Anki.
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Some people seem to consider extremely detailed memory to be an intrinsically meaningful purpose on its own merits. I'm quite suspicious of this in most cases, but sure, World Memory Championship winners seem to find that a powerful source of meaning.