The thing is modern Mercedes' and BMW's for a long time have been pretty stout cars. Adding in the newest technology (read electronics) are always something to be concerned about in any car though but in terms of overall chassis, engine and transmission design are not something that people should really be afraid of. Sure, maintenance costs for a German vehicle are pretty high but so are Lexus vehicles to their own extent.
Leasing a Mercedes or a BMW is certainly not the worst idea. There are also people who do buy Mercedes and BMW's but those are the ones who are not planning to get a new car after their lease deal is over. In fact, they were never really bad vehicles to start with. There are some exceptions though.
It's not like these brands have been perfect though and I understand people's fears when it comes to some German vehicles and for good reason.
When it comes to BMW, their NA V8 and the NA V10 M cars were a ticking time bomb, some of the older automatic transmissions that BMW used as well were pretty weak, their dual-clutches are pretty clunky around town and have been reported with some issues and electronics do exist as well, but honestly, you won't have any of those issues. The NA V8 and NA V10 engines are yesterday's news so you don't have to worry about that. The ZF 8-speed transmission that's in all modern BMW's is simply quick, comfortable, and bulletproof reliable. The only thing is electronics but after some time that stuff gets ironed out too. Also you have the warranty to protect you if some ungodly event happens by accident.
As for Mercedes, like BMW there isn't anything really bad to say about them unless you're talking about some of the electronics again or some older cars that were absolutely a pain in the a** to deal with in terms of reliability. Mercedes also does iron out their issues over time. It's not bad. And if something bad happens, you have a warranty.
Mind you, these issues can happen to Lexus vehicles as well. In the modern era of the automotive industry, manufacturers are converging when it comes to reliability. We're reaching to a point where a lot of elements of a car be the car's chassis, engine, transmission and overall driveline are solidly built. To this day there are still exceptions to the rule like FCA (Fiat, Alfa Romeo, Chrysler, Ferrari, Maserati), some Ford and Chevrolet vehicles, Aston Martins (which without Mercedes' help they would be worse off), Nissan, etc..
Just give some love and respect to a Mercedes or a BMW like you would to your Lexus and they stay pretty solid for quite a long time. They won't last as long as an old Toyota which lasts for like 567 years but heck you're talking about two completely different things here. I feel like people do confuse with maintenance costs with reliability a lot and those maintenance costs won't be so bad if people aren't idiots around cars in the first place.
As for other German brands, Porsche is almost the most reliable brand out there full-stop so there's nothing much to worry about unless you want to do maintenance. VW isn't that bad but they're known to have issues in some of their cars as do Audi's.
