The Paladin was really missing from the core rules, kind of expected maybe from core rules that made casting channeling spells in metal armor impossible. If you did not run a world with a setting calling for the paladin archtype you might not have missed the profession. Having core rules that made metal armor and channeling spells impossible to mix is IMHO flawed in itself for a game that was aimed at supporting generic fantasy.
Except that "generic fantasy" is, in this case, purely AD&D... for which, back when RM2 was created, the Paladin wasn't even a core profession per se (it was a sub-class of the Fighter)! Saying that, about the matter, RM2 was "flawed" because it could not predict that, 30 years later, the Paladin would become popular in fantasy settings based on AD&D is a bit... unfair.
RM2 is not 30 years old even this day
True. But since RM1 is (roughly speaking - some parts slightly more, some slightly less), and RM2 was mainly adding onto RM1 various options that statement was a reasonable approximation. Point doesn't change.
and even if we go with the notation that AD&D defines fantasy
You missed the point. The point is that the only way your statement makes any sense is if "generic" is read as "D&D-derived". The idea of a holy knight predates D&D and is not uncommon. The idea of a
spell-casting holy knight is a D&Dism with little, if any, basis in fantasy not itself influenced by D&D. If you wanted a literary case of channeling use in metal armor, Elric is a far better choice.
Sorry but there is simply not any good excuse for outright forbidding channeling spell casters from having armor if the goal is supporting generic fantasy.
Given that "generic fantasy" only ever means one of two things: (a) D&D-derivative, or (b) formulaic pseudo-Tolkienesque crap, this hardly seems like this would be a worthwhile goal. However, RM seems fairly supportive of the second type of generic fantasy, having the necessary races and various Middle-Earthish touches.
If you want to cover the fantasy genre in general, you can't just have a rules set, you need a rules-construction kit and you'll want Fantasy Hero instead of Rolemaster -- although as more and more companions piled up, Rolemaster became more and more of a kit rather than a game. Or you can use your judgment to tweak Rolemaster. Remember that Rolemaster is an old-school game, written when it was assumed the GM was capable of independent human judgment.