Interesting point, but... have you learned that from the designers or did you come up with that from other sources?
I am curious if it is RAWish or your interpretation.
Actrually, it has been explicitly stated (in the forums at least, maybe in a sub rosa as well) by the line editor that the "no general MR for non hermetics" is a conscious design to keep the Hermetics as the big fish in the pond. Parma is the achievement of the Order. You can have the Order exist without a common hermetic magic system, but not without Parma, so keeping parma as the great thing it is is what it is all about 
Xavi
Phew... the Hedewizards/magic humans appearing in earlier editions adventures (The Stormrider, Pact of Pasaquine) have might of about 10-15 even if they can pull of stuff like a low to average magus. It is not that hard to overcome their resistance compared with an average magus. That was what PM originally was about and that invention stopped wizards from killing each other. In the adventure old "Bats of Mercille" by the Mark Hagen it is hinted that Hermetic magi are in fact small part dragons... that is why they have the Gift.
For people you like the gamey approach the line editors choose for general MR in ArM5 to keep the Hermetics the big fish I have another suggestion; why not forbid hedgies from wearing armor, use weapons larger daggers and restrict them from using more than two rings at a time? No! We don't need an explanation for this at all, skipping these restrictions was just some more of Bonisagus great inventions. Remember - just making their MR much lower than PM isn't enough
I'm sorry, was there a point to this post?
He's saying its a bit silly to just handwave the only magic resistance for Hermetic Magi as a gameist perspective. Why not just toss on other random restrictions and say only hermetic magic can do it?
I can see that, I'm just confused as to his point.
Those restrictions don't make sense in setting.
In setting it makes perfect sense that having magic resistance requires you to know how to get it.
The parma magica is what knowing how to get it is.
But hey, why not complain about the fact that hedgies can't teleport everywhere by blinking? Or that Magi can't blow up the whole planet?
As a side note: Folk Witches can have magic resistance. They can get familiars, and if those familiars have might they can use that resistance.
Note, the planet is assumed to be pure metal since that is what the core becomes.
This means that to target the planet you need a spell with +23 magnitudes for size, and a base of five for destroying metal.
You also need a rego requisite to cause it to explode. And a final magnitude for touch range.
Therefore the spell we want is:
Blow up the Planet: PeTe(re, ig) 120
Range: Touch, Duration: Mom, Target: Ind, Ritual
This spell explodes the planet with a burst of natural motion. The strength is sufficient to overcome the gravitational binding energy (or in the medieval paradigm disrupt the cause for natural motion downward). While only earth is directly effected the force will destroy most everything else and scatter it across the sphere.
Base 3+2 metal+1 requisite+1 Touch +23 size.
To create this spell you will need a team of magi with high MT+int+virtues, led by someone with a high leadership score and PeTe(re ig) totals. Difficult to be sure, but not outside the realm of possibility. You could have someone with 12ish leadership a specialty, and a bunch of helpers with 12+ in the assistance totals. It might take a few years, but you could pull it off. Alternatively you could steal an angel's "magic" ability and simply generate a 120 level spell.
Do they? If they follow the same rules as Hermetic Magi sans cords they certainly do. Which would ironically immediately get a join or die order to all folk witches. You can also get MR by going on a pilgrimage to pick up a relic. Also I find it weird that any hedgie who gets on God's good side gets threatened. "Hey God likes that guy. Lets go blackmail him!"
That's cool. Since you are so good at figuring out magnitude boosts for Sizes, what would a spell that blows up the moon look like? Assuming that the Lunar Limit had already been breached, of course.
Matt Ryan
Ah, but we don't actually need a spell for this - just a charged device with a single charge (unless you subscribe to the idea of worlds within world I suppose).
Then level 120 is ... a fairly decent Verditius, with a familiar and probably a pair of apprentices.
Or a relevant magical focus I suppose.
Don't forget to get that Ritual Items Breakthrough from Folk Witches. And get your MT high enough to use 24 pawns of vis in a season. Noble's Parma on whatever reduction Verditius might get. 
"Mercurian Magic" would reduce the Vis requirement to just 12 pawns and could, arguably, circumvent the seasonal Vis restrictions as well!
It's called "irony"
That's another thing to point out how incredibly stupid this rule is. What used to be game mechanics (that is Magic Resistance) is now a inconsistency for the sake of nerfing hedgies who forgot to buy pets or didn't think of getting enough different charms.
Hermetics get MR from their familiars. However, it is a long shot implying that all familiars out there would do the same.
In any case what allowed the formation of the Order was parma, but not because it granted MR (we all know that the MR provided by parma is quite lame in front of a penetration specialist) but because it nullified the bad effects of the Gift. That -3 and crook social appearence
Why?
It's only a ritual because it's high level, and the rules excplicitly allow me to invest that into an item.
Right there on p. 96 of ArM5.
Again, Why?
Charged item, remember? They have no Vis cost 
But it's not a rule. Any Gifted person could learn Parma Magica. And there are potentially a few other ways of getting a (usually small) general Magic Resistance.
However, a part of the setting is that the Order of Hermes has tried to, and has been largely successful at, keeping the Parma Magica a secret. Precisely how successful the Order has been at keeping this secret, and whether it remains a secret, is a matter for your own troupe to consider.
Oh, I always for get that the lvl50 does not apply here. 
If the Charged Item used the Folk Witch Breakthrough, it'd need the pawns of vis, hence the doubled mistake.
It destroys the planet. That's a "major effect" that forces it into ritual status regardless of level and/or itemhood.
The folk with breakthrough is precisely to put ritual magic into magic items.