This is the car that Brian Wilson was thinking of...a two door Biscayne with the posi-traction 409...lol...
But to your point, the SUVs are today's grocery getters and the grocery getters of the future...Doubtful Impala will survive, but I think GM is ahead of the game with Buick and Cadillac....Ford has nothing to compete with Mercedes or BMW, but GM does...
I think Buick and Cadillac will survive any markets or changing demands with full size luxury sedans, but they will get very pricey.
SUVs are popular in large part only by comparison. Once lower cost sedans like the Impala are gone, and all you can get are SUVs, etc., their days of popularity will eventually wane, especially if they are all you can buy...
Bottom line though I believe is as much as car-makers want to build what sells, they can't start and stop models fast enough to please the majority. They'll always be behind the curve....
Human nature shows what people want....and what is that?...It's what they don't have is what they want....Once sedans are gone, they'll want them back.