I agree that merchant adoption needs to get a lot better (e.g., see the education aspect on which I posted earlier today).
But note that the metric you observe doesn’t really say much about who does shielded transactions. It just tells you how funds enter the shielded pool. Even in the hypothetical world where everybody used z→z for all their payments and fund movement transactions, you’d still see block rewards sent to t-addresses and then shielded (by miners and ECC) using t→z transactions.
Also, you can actually expect this metric to improve when the Shielded Coinbase ZIP gets merged and activated, since using block rewards will no longer require t→z shielding. But this won’t reflect any merchant adoption…