No, how much money or effort Google spends is irrelevant to the value they provide.
If Google came up with a magical technique to get rid of all malware on their app store that didn't cost them a dime, you could still argue that they're adding value to justify the 30% cut.
If Google spends a billion dollars to get rid of all the malware on their app store, and it doesn't work, and using the app store is not significantly less dangerous than sideloading apps, then they're not adding any value to justify the 30% cut.
Note that I don't think Google's efforts are completely fruitless. At the very least, they seem to help with taking down duplicate apps, and that's something. But whether or not moderation is valuable only depends on whether they succeed or not -- their effort is irrelevant.
If Google came up with a magical technique to get rid of all malware on their app store that didn't cost them a dime, you could still argue that they're adding value to justify the 30% cut.
If Google spends a billion dollars to get rid of all the malware on their app store, and it doesn't work, and using the app store is not significantly less dangerous than sideloading apps, then they're not adding any value to justify the 30% cut.
Note that I don't think Google's efforts are completely fruitless. At the very least, they seem to help with taking down duplicate apps, and that's something. But whether or not moderation is valuable only depends on whether they succeed or not -- their effort is irrelevant.