I have never been told a part cannot be ordered, other than for the reason it is no longer available.
I have my old and my current A-classes serviced at Mercedes, for the past 6+ years, and never heard anything like "we don't have such-and-such".
And mines are not exactly brand new cars
- the old one was Y-reg, this one is 06 ...
I also bought an awful lot of parts from Mercedes dealers, both local and online, particularly for my current car, doing all kinds of retrofits and mods, and they always obliged (if I could afford it).
I understand your frustration, but the conclusions you've arrived at are simply not accurate.