The capital gain is the profit, the collateralized lending is the transaction completed to realize that profit. It's a logical extension of accepted understandings of those terms and easy to imagine coherent legislation to implement.

You don't like the idea, that's fine. But it's simply not true to claim that it doesn't make sense and you haven't been able to articulate any inconsistency. Just saying "nuh uh that's not profit" is pointless. We all know it doesn't constitute realized gain in the existing system of laws, but OP and others are suggesting it would a be a sensible way to tax the extraordinary benefits that the ultra-wealthy take from their appreciated assets. It's been explained to you politely and with sources, if you have nothing more serious to add to the conversation I'm done giving you the benefit of the doubt.