No, it does have everything to do with level bonuses and is, in fact, fixed by RM2/RMC level bonuses. A 20th level fighter picks up a new weapon and, thanks to their +60 level bonus, they are immediately at least reasonably able to use it. They aren't as good with it as a warrior who has specialised in it since level 1, but they are no slouch either. A few levels of development and they will eclipse other, casual users, quite easily. A 20th level thief starts getting involved in an aspect of subterfuge they never really practised but thanks to their super-human knowledge of skullduggery, they again, jump ahead. A magician with decades of magical experience starts learning some new magical skill that falls within their realm of learning and they are able to put all their past experience to use and boom, they again, don't start out as a neophyte and easily catch up with the casual user. No other edition of RM has really handled that kind of holistic skill improvement as well as RM2.
Now, rapid development does sound like a good mechanic.
Ok, let's take a look at 20th level fighters in all three versions. I'll also do 10th level for comparison. I'm going to ignore stat bonuses since they will be the same for trained and untrained skills, so just looking at rank bonuses and profession bonuses. There are also some details about ranks from 0 level, adolescence, culture, etc but for simplicity I'm going to ignore that too.
RM2
In RM2, 20 ranks at 10th level gives you a rank bonus of +70. The level bonus gives you +30. So that's a total of +100. If we imagine a fighter with a sword who picks up a dagger for the first time, they have a rank bonus of -25 for a total of +5; effectively you are at a penalty of -95. If you had just two ranks (one level's development), you'd have a bonus of +40 (effectively you are lagging by 60).
40 ranks at 20th level gives you a rank bonus of +85, with a level bonus of +70, for a total of +155. With that dagger and no ranks, you're at +45. With two ranks, you'd be at +80.
RMSS
In RMSS, we can assume the 10th level fighter has 20 ranks in the category (e.g. 1H Edged) and 20 in the skill (e.g. broadsword). That yields a rank bonus of +50 +30 = +80. Fighters also get a profession bonus of +20, so that's +100 (same as RM2).
If this fighter picks up a dagger for the first time, their category ranks apply (+30) but not the skill ranks (-15), so they are at +35. If they had just two ranks in dagger, they'd have +6 +30 = +36, plus the profession bonus, for +56.
So, in RMSS, if you switch to a similar weapon, untrained you are in a much better place than the RM2 fighter (+35 vs +5). With two ranks, the RMSS fighter still has the advantage (+56 vs +40).
However RMSS is less forgiving for a major change. If the fighter picks up a mace, their category ranks don't apply anymore, so now they are at -10. With two ranks in the skill and category (one level of development), they are only up to +30. So RMSS fighters are more flexible within a category, and less flexible outside of one.
How about 20th level? 40 ranks gets you +35 (that's the maximum for the category) +65 = +100, +20 professional bonus = +120. At this point the huge unlimited bonus for the RM2 fighter means their bonus in a trained weapon is quite a bit higher.
Switching to that dagger, the 20th level RMSS fighter is now at +35 -15 +20 = +40. With two ranks, +35 +6 +20 = +61. These bonuses are lower than the 20th level RM2 fighter, but the starting bonus was lower in RMSS too. If you consider how much worse off the RMSS fighter is compared to an equal level foe with their best weapon, the RMSS fighter with no ranks is lagging by 80 (i.e. as if they were fighting with a -80 penalty), and with two ranks by 59. The 20th level RM2 fighter was lagging by 110, and with two ranks by 75. So, the RM2 fighter is suffering a greater effective penalty for switching to this similar weapon.
However if he picks up a mace, that 20th level fighter is now at -10 (effective penalty of -130) and after two ranks +30 (effective penalty of -90). So with a very different weapon, the RMSS fighter is worse off.
RMU
In RMU, at 10th level 20 ranks in a regular skill gets you +100; as a fighter you would almost certainly have a +5 knack on top of that for +105. If you switch from a broadsword to a dagger, your skill is still +105. So within the same category of weapons, you are far better off than either the RM2 or RMSS fighter.
If the RMU fighter switches to a mace, you can keep using your Melee: Blades skill, but with a similarity penalty of -50. That puts you at +55. Not only are you better off than a RM2 or RMSS fighter with no skill in their weapon, you are even better off than one who has put one level of development into the new weapon skill.
At 20th level, you start with +140. That comes in between RM2 and RMSS. As before, you can switch to a weapon in the same group at no penalty. Switching to that mace, you are now at +90. Again, you are better off than a 20th level RM2 or RMSS fighter who has put one level of development into the skill.