Understand this First The main reason iam against this, is its simply going to promote solo play more than party play, making most hard mode instances irrelevant in the process( not all like i said on my first comment)
Like you said
there are drops there THAT CAN ONLY BE FOUND THERE
That's true for now , if you make mythril breakable into ores that wont be entirely true any more.
Sure crystals and stones are still obtainable only from instances , But you have to consider why people farm Hard mode instances mainly at present
1.Mythril Ore (very Demanding since used in many stuff, need alot for every one ,we get from all hard instances. one of the main reason why people party up and do hard mode instances right now)
2.Shimmering Crystal (need a ton for every one but can easily farmed compared to mythril ore )
3.various stone from hard mode instance (demand depends on stone types very few farm for this right now mostly people go for Sparkling Opal) or Aqua Sapphire)
4.Crimson Magic Crystal (this will be the most sort after item in long term requiring intense farming but no cares about this at present, obtainable only from normal mode izlude/sunken ship as of now , so lets ignore this )
SO how Hard mode party works now
With a party of 2-7(including dualing) Most people run hard mode GTB ,orc ,izlude and ant hell once per day or multiple times a day if they have enough alts . they do this mainly for mythril ore in mind, this is repeating content but atleast your doing it with party which makes it fun.
They try to do as much instance they possibly can to farm maximum mythril ore
Shimmering scale is like secondary drop
(No one does hard mode sunken ship as of now since its hard with current gear progression)
So what happens if we make mythril degrade into mythril ore
1.Major loss in interest for doing hard mode instances for many people who wants mythril ore more than shimmering crystal since now you can farm mythril efficiently from fever maps and break them.so less party for instances
2.So only major reason for people to party up and farm hard mode instances now will be for Shimmering crystal , ores.They will end up doing only Hard mode Gtb and may be just orc ignoring ant hell and izlude.(since izlude,ant hell takes way more time per run )
3. Or worst case they will just end up soloing GTB hard mode for farming shimmering crystals with bunch of alts. It makes sense to do that since more crystals for you than sharing it with party and can be done easily with dual client.
4.People who need stones from orc, izlude or ant hell will never get or rarely get instance party and the guy who needs stone from sunken ship has 0 chance to get instance party.
making Hard mode anthell , izlude and sunken ship pretty much dead content in the process.
Since they added mythril to desert fever mobs/mvp which is a bit easy to farm than mythril ore Surely there is imbalance now no one denying this.
Possible Solution to mythril/mythril ore imbalance
1.Increase Mythril ore drops significantly for hard mode instance for party of 5 or more. this is ideal since it requires atleast 3 people with dualing, anything less can be easily exploited.
(Right now with 7 people you get like 2-5 mythril ore drop , if you solo gtb hard mode 1-2 ores do the math.)
drops should also scale heavily according to instance level and difficulty (now every hard mode has same drops expect the stone ) else the higher level instances will be less relevant.
2.You could Make mythril degrade into mythril ore this will cause huge negative effect to most hard mode instance as said above, to balance this you might need to heavily buff mythril ore drops with scaling for hard mode gtb, orc,ant hell while adding Crimson Magic Crystal to hard mode izlude, sunken ship to keep them relevant.