I think you just aren't thinking this through. If I take a Fighter and rank up my combat abilities fast and cheap, then switch to a pure caster and level spell abilities fast and cheap I'm going to be significantly powerful than either of those professions at an equal number of levels.
In fact no, you won't be. You might have a broader selection of decent skills. But the pure caster will be only as good as others HALF his level. And the fighter as well, though that may not be quite as relevant.
Half his level. Half.
No, he won't be. This would only be true if skills costs were the same between the two professions.
I'm right, you aren't thinking this through. You're stuck on levels, but RM isn't really about levels, it's mainly about development points. It doesn't matter near as much what level a character is as how many skill ranks they have.
This is assuming when he would have become a 6th level pure arms user you're just continuing to progress using the 6th level Exp requirements to make it be a '1st' level pure arms user.
Let's take Bob the fighter. So let's just keep this low level and say 5th level:
As a Fighter he'll have paid 1 and 3 development points (total of 4) to get two ranks of weapon skill per level vs a cure caster 1 rank at a DP cost of 9.
- At a 5th level fighter he's paid 20 points to get 10 ranks of weapon skills vs the 5 ranks that would cost him 90 if he were a pure caster. He's paid 20DP less for twice as many ranks as a pure caster.
- If he bought spell levels he'd be pay 25 point per level for just OPEN spell lists. So as a Fighter that would cost 250 DP for 10 ranks in one spell list alone. But he's not going to do that, cause he's not an idiot.
Now he become Bob the mage at 6th level.
- He's not going to pay 9DP per weapon ranks (which he could only do once per level) cause, again he's not an idiot and has 10 ranks already, which is far better than most pure casters.
- Next, he's developing Base spell lists, more powerful than Open and Closed, for 3 ranks per level at 3 DP per rank. So we'll say he levels four Base spell lists to 15th level potentially for 180 DP. So he's paid 80 DP less for three times as many ranks in four times the spell lists.
At 10th level Bob has saved has, at minimum, twice as many weapon ranks for 28.5% the normal cost and access to six times as many, more powerful, spells for 72% the normal cost.
And we haven't even talked about Body Development, Armor skills, Combat maneuver skills, Power Point ranks, etc, etc.
If he as good at Combat as a 10th level pure arms user or at Casting as a 10th level pure caster? Nope, but he's nowhere near 'half' the character incredibly more versatile.
I don't think I understand what you're saying here. Fictional in-game characters wouldn't concern themselves with game balance?
Yes. They have no concept of that. And the universe does not follow the rules of game balance either, unless the gods designed your fantasy world that way. But even if they did, see above: No issue.
But you're talking about changing game system rules (and the resulting imbalances in it), not the in-game fictional world. Not at all the same thing.
I don't believe a second that ChatGPT 'knows' Rolemaster.
This could be a separate thread. 
ChatGPT is not intelligent. It doesn't 'understand' anything. It scraps the internet for information and regurgitates that information based on perimeters you set for it.
It's like the computer 'Waston' that played Jeopardy on a bigger scale. It doens't know anything, it just searches a database for answers.
You could tell ChatGPT: 'Write me an essay on why the Easter Bunny is real' and it would.
