That is preposterous, and you're not getting my point and understanding why I use the M3 as an example, because:
a) the M3 is as much, if not more, a compromise between sports car and practical upright sedan with its own limitations (which it is - it's no 911) than a performance wagon vs the sedan version of the same car. In fact, BMW is probably insanely close to making an M3 touring wagon with every generation, but don't make it because it won't make significant amounts more money than it would cost to develop and crash test and advertise in its respective markets, that and they couldn't sell them in America where everybody thinks they need a battleram to drive on the streets. When they do make an M3 wagon (and they probably will break down and make one eventually, there's absolutely nothing stopping them besides the profits) - it will run a couple tenths slower than the coupe and sedan, naturally, and nobody will care, because it will have double the utility. It will follow logic there. Everybody will understand why they will make the M3 wagon, and people who want one will buy one.
b) You act as though people just can just add on a beater whenever necessary - that basically only works if you plan to live in a place where nobody else really wants to live and you have free reign on parking. Otherwise, you can be of good means but still be forced to pick a couple/few cars to own at a time. The only house I have seen in Tokyo I was interested in actually owning cost $3m, and yet it had car spaces for 1 compact and 1 midsize in the garage. That isn't even common here, most houses have 1 space. There's no need to own a beater as it's the middle of the city and stores will wrap everything so that your car doesn't get dirty or marred, but utility is a boon, and so is the possibility of having 7 seats. When you're living in confined areas you minimize the footprint and maximize the quality. The E63T wagon, the M5T, the RS6 Avant, they fill this void as the top model in their size class. The E92 M3 is actually a pretty good car, all things told, but it's still a coupe with a mostly useless backseat and a conventional trunk, which makes it fairly useless for anybody but a bachelor. it doesn't get very good fuel economy, which yields it negative points considering it's size and performance stats.
c) The AMG E63T has a ton of cachet in hyper developed/matured markets like Europe and Japan (I wouldn't consider America in this because the monied centers of the country tend not to be hospitable to private drivers, barring the Hollywood people and their leases), because a) it tends to be a personally-owned private vehicle, vs a business-owned car like an S-class, b) being the highest model in its range, c) rarity value, and d) AMG was once the purveyor of insanely expensive to-order cars that tripped $300K for the E-class Hammer and that reputation exists wherever they were sold back then, and while the prices have come down due to the full MB takeover and volume sales, they're still built similarly and sold at a third of the price now. BMW M and Audi quattro have never come close to that level in their history.
d) the M3 is good, but not a supercar. There are cheap American muscle cars running pretty close to what the M3 does, at half the price. The E63T - it's a 550hp wagon with seats for 7 and performance on par with your beloved M3 coupe, what else does it need to prove? The CTS-V wagon is close, but it's so ugly that I want to pretend it doesn't exist.
there's far more than this, but this is very tip of the iceberg. There's no more reason for a modern Range Rover to exist in the SUV category as there is an E63T in the wagon category, or an M3 in the compact category.