Mercedes did with Maybach what Toyota did with Lexus, Nissan with Infinity, Honda with Acura, and Hyundai with Genesis - have separate brands for their luxurious cars. Maybach is luxurious, Mercedes is not. That's by design...
There are very many other examples: VAG brands of Seat (cheap), VW (midrange), and Audi (upmarket). Land Rover and Range Rover are another example. And when Ford owned Jaguar, it kept the brands separate, in spite of the X Type having Mondeo underpinning. Etc etc.
The concept was actually invented by the Americans many years ago, and lives on to this day. Several dozens American car brands were made by only the three big automakers, to be sold with different trim levels and at different price points under different brand names. Chevrolet, Pontianc, Cadillac, Buick were all made by GM. Ford, Lincoln, and Mercury were made by Ford. Chrysler sold Chysler cars as well as Dodge, Plymouth, and Fargo. This followed the consolidation of the market and disappearance of the small independent automakers.
And it's not difficult to understand that a Bentley with a VW logo (Pheaton) cannot be percived by buyers as luxurious.
So I don't think MB wants the Mercedes brand to be luxurious... if they did, they would have made the A, B, C and E under a different brand name, like every other automaker did over the past 100 years. Yes, also the E, because you can't call an E220 with poverty spec and plastic seats 'luxurious', even if it passes as an Uber VIP (and I am saying this as someone who owns the model that is Europe's favourite taxi - the C-Class).
As for warranty and reliability.... I don't think these are related to being a luxurious brand, instead it's a totally separate parameter.
If you look at the JD Power reliability surveys, you will see that both the pole position and the last position are held by luxurious brand. The most reliable brand is Lexus, the least reliable brands are Range Rover, Jaguar, and Tesla. ThThere's st no correlation in the market between luxury and reliability.