If you truly never enter credit card info on the web, or truly have zero expectation that other tabs shouldn't be able to spy on that credit card info when you

If you truly never enter credit card info on the web, or truly have zero expectation that other tabs shouldn't be able to spy on that credit card info when you do, then frankly you're so outside the range of normal people's expectations and behaviours that I don't think basing security architecture around what you do or don't do makes any sense whatsoever.