This isn't even going to work in the long run anyway. Imagine being the car sales person. It makes you sound like a cheap used car salesman trying to nickle and dime the customer while selling "luxury".
The whole idea will be perceived for the pettiness and ripoff it is. Especially when no one will even know you pay the extra a month so there is zero status symbol with this.
No way this is around in a few years. They will lose more sales than they will ever make up with the monthly fee.
Markets which require billions of dollars to compete in are not truly competitive. They may be competitive in a regulatory sense, but not in a practical one. And these markets have huge amounts of regulation as well that limits competition on top of the financial limitations on competition.
Slim margins say nothing about competitiveness. You can lose money in uncompetitive or competitive markets. You can have huge margins in competitive ones.
Also I don't think the 10.5% profit margin of Mercedes is low - the average for the S&P 500 is 11%.