I think extension blocks could be optional, and it could be many different extension blocks with different functionalities like Confidential Transactions or smart contracts. Only the interested nodes would enable this extension blocks, the rest would see only the classic blockchain without extension blocks. So it's not a matter of "old" and "new" nodes, all are updated nodes with extension blocks enabled or not. The only ones that need to understand the protocols of all existing extension blocks are the miners, which must verify that transactions from "anyone-can-spend" to a "classic" address are legal.

Classic address to classic address: as always
Classic address to extension block address: transaction to "anyone-can-spend"
Extension block address to classic address: transaction from "anyone-can-spend"
Extension block address to extension block address: it doesn't see it because it doesn't download the extension blocks, only the main blocks.

All coins that are in extension blocks are also in the "anyone-can-spend" address of the main blocks, so basic nodes are aware of the total number of coins. It's totally safe.

So for the particular case of Confidential Transactions, it would work as explained. The CT transaction details would be in the extension block which could have the same size as the main block so the total size of the blockchain (main blocks + extension blocks) would be double.

With this method bitcoin could add new features without losing the "store of value" property, as the base protocol never changes. Again, maybe I'm missing some technical detail here, I'm still learning 😊