I believe they retire because they lose their commitment not their speed, they both are good drivers as they are gp2 champions. Can't you possibly accept there is a chance that rosberg is the better driver?
The 2 are not mutually exclusive my friend, and drivers retire for a range of reasons. You will find that most F1 drivers lose their speed as they approach retirement age.
As for Maldonando/Hulkenberg, we do not know how good they are in F1 terms. Plenty of drivers in the past have come to F1 as champions from lower formulas, but then went on to achieve very little in F1.
Rosberg is better at this point in time - that much is clear.
Is Schumacher as good as he was 10-15 years? no i don't think so.