I think you have some valid criticisms, but we have to take baby steps here.

As you mentioned, multi-generation homes are not the norm in the US. If there is a change in that norm, it's worth investigating why that's happening, before we conclude that we need to adjust metrics around it.

Purely anecdotal, but every multi-generation home in my (American) family is the result of disability or student loan debt - multiple generations live together, but that doesn't mean they want to.

Also remember that this new metric is meant to be more informative on wealth, generally. Those other generations living in the home don't typically share in the wealth the home confers, not until someone dies at least.