Yeah, it would be more useful to look at the number of new xpub/ypub/zpub keys over time. But even then one person can still own more than one pub keys.
Discussion
The problem (or the benefit) is you wouldn’t want to share your pub keys for the sake of privacy.
So if i understand it correctly, you don’t have an onchain metric that can tell you how many xpubs (zpubs …) own how many btc? Because that information is never included in transactions? 🤔
Basically yes, a btc address is a hash derived from the pubkey, which is itself derived from the private key. Transactions include only sending and receiving addresses + signatures (derived directly from the private key).