I agree with you, but a reasonable case could be made for viewing standard of living as material access so I think that's where a lot of us talk past one another.
Also American society is pretty disdainful of security, historically.
Isn’t housing part of that material access which has become harder to maintain and access by more and more? People living paycheck to paycheck to pay huge rents, people living in tents and so on seems to indicate some decrease in the standard of living.
The argument against this would be things like homes being larger now and housing standards having risen (even slum units have things like fridges and electricity). I don't think those are compelling arguments because often going without those things is not allowed/not an option, but that's what the other side's argument would be.
The people that live in tents generally used to live in state mandated mental institutions. Their standard of living decreased (well depending on how much you weigh autonomy), but that’s not really relevant to the standard of living for the middle class.
Also American society is pretty disdainful of security, historically.