Every vehicle that passes the warranty point, guess what? That car manufacturer knows why the warranty is good for that amount of time. So if you decide to keep that that particular car, be ready for maintenance.
And even if you do a routine maintenance, if the parts replaced are done with cheaper parts (read CPR made or similar), then expect them to fail rather sooner. I own 3 BMW's, they all attend TLC, and have no issues.
Concerning the alternator, what I have done, is rebuild them - it's not hard, the parts are the fraction of a cost of a new unit, but again, you have to know what bearings to buy - the CPR are weak and will break quickly. usually the brushes/regulator combo goes at the 100k mile mark, which is really easy to change - you don't even need to pull out the alternator on the i6 models (the v8 needs pulled out).
For the cooling system, if one flushes the coolant every 2 years, the rubbers & plastics don't get tainted as quickly.....
There isn't such a thing as a bullet proof car that doesn't need maintenance and will go and go and go, just drive and drive and drive. And the later the date of manufacturing, I think the more problems these cars have. Just think of all the electronic hip gizmos that are out now - in Car & Driver they were talking about that intelligent feature in the new Infinity and they were scared once the vehicle took over: they couldn't brake, they couldn't steer - scary at best. They said the brake pedal gone stiff, same with the steering wheel. I'd rather get a 1600..........

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.